2. Our coasts and estuaries are affected by a changing ocean

Indigenous coastal, estuarine and marine species and ecosystems are valuable natural assets. They provide vital protection from coastal hazards, store carbon, support biodiversity, and underpin cultural and economic wellbeing. Yet these environments are increasingly at risk from the compounding impacts of climate change and human development. Critically, the effects of a changing climate and human activities on the ocean lead to changes in our coasts and estuaries as well.

This section examines how climate change compounds other pressures on coastal and marine species, habitats and ecosystems. Sea-level rise intensifies erosion and flooding risks in some areas and contributes to coastal squeeze, where natural habitats are trapped between rising seas and built infrastructure. The increasing frequency and severity of extreme weather events – including rain, winds and waves – add to these pressures, damaging ecosystems and infrastructure. Acidification is also more intense in coastal waters, due to increased carbon dioxide levels. Other pressures faced by coastal habitats and ecosystems include marine heatwaves, the increased mobility of invasive species, and marine diseases. All of these pressures are having devastating impacts on indigenous populations, and those impacts are predicted to continue.

These changes threaten the ability of coastal and estuarine systems to support the lifecycle of many species. They also undermine the ability of these systems to support cultural values and to deliver essential services such as carbon sequestration and natural coastal protection.

Sea-level rise threatens coastal ecosystems through inundation, flooding and erosion

• Sea-level rise is projected to lead to a cascade of negative impacts on low-lying coastal areas, including inundation, increased coastal erosion, saltwater intrusion into adjacent freshwater, and drainage issues. These effects can damage infrastructure, displace communities and disrupt ecosystems (MfE, 2024).
• Aotearoa New Zealand could begin to lose intertidal flats as early as this decade if rates of relative sea-level rise exceed about 5 millimetres a year in areas with limited sediment supply (Swales et al, 2020). In 14 estuaries, intertidal areas are predicted to decrease 27 to 94 percent by the end of the century in response to projected sea-level rise of 0.2 to 1.4 metres, if landward movement of the intertidal zones is restricted (eg, due to rapid rises in sea level, or the presence of flood and coastal defences) (Mangan et al, 2020).
• In the Hawke’s Bay region in 2024, saltmarsh and seagrass ecosystems covered a total area of 1,476 hectares, of which 87 percent was saltmarsh habitat. Sea-level rise is estimated to reduce saltmarsh extent by 11 to 12 percent by 2050, and by 25 to 31 percent by 2100, depending on the sea-level rise scenario (Bulmer et al, 2024a).
• Predicted changes in tidal inundation and increased water depth due to sea-level rise will reduce light availability. This will negatively impact species such as seagrass (Capistrant-Fossa & Dunton, 2024; Lundquist et al, 2011), which plays an important role in supporting many fish species (Berthelsen, 2024).
• Rising sea levels increase the risk of erosion from waves, storm surges and high tides. This risk can be exacerbated by extreme weather events. For example, in February 2023, Cyclone Gabrielle caused 5 to 10 metres of erosion at certain beaches across parts of Northland, Auckland, Coromandel and Bay of Plenty (Coastal Change, nd).
• Coastal erosion – the wearing back of the land – is an important hazard for coastal communities and ecosystems (MfE, 2024). Shoreline mapping of open coast beaches and soft cliffs since about 1940 suggests that 40 percent of the coastline mapped is eroding, 42 percent is accreting (advancing seaward) and 18 percent is either stable or unresolved (Coastal Change, nd; Tuck et al, 2024). For example, Kaipara Harbour’s North Head is one of the top hotspots of coastal erosion and accretion around the country, and it is eroding by around 6.4 metres a year (Coastal Change, nd). Coastal erosion also threatens people’s property (see section 3).
• Sea-level rise and storm surges threaten coastal ecosystems and freshwater species by moving saltwater farther into coastal freshwater environments, altering their salinity (Lawrence et al, 2022; Neubauer et al, 2013; Schallenberg et al, 2003).
• Rising sea levels have led to a loss of nesting sites for various shorebirds and may put more species at risk (Keegan et al, 2022). For example, New Zealand’s rarest indigenous breeding bird, the tara iti/fairy tern (Sternula nereis davisae), has an adult population of only 40 birds. This critically endangered species nests on beaches and is facing increasing threats from climate change impacts such as sea-level rise and intensifying storm surges (Brumby et al, 2025).

Changes in rainfall, storms, waves and winds threaten coastal health

• Climate change is leading to more frequent heavy rainfall events in some areas. This may cause landslides and soil erosion and intensify land degradation, especially in areas with exposed soils or non-native vegetation (Neverman et al, 2023). The resulting sediment run-off from degraded catchments flows into estuaries and coastal zones, reducing water quality and smothering sensitive habitats (see Our environment 2025).
• Storm surges and powerful wave forces can physically damage coastal habitats. For example, mussel beds can be dislodged or broken apart, especially when already thinned or loosely packed, with the result that mussels die if they cannot reattach (Hunt & Scheibling, 2001).
• Changes in wind and wave patterns are predicted to alter sediment movement and coastal upwelling of cooler nutrient-rich ocean waters in some places. These are important for coastal productivity, including fisheries (Bell et al, 2001; MPI, 2021a).
• Cyclone Gabrielle in 2023 highlighted the impacts that severe weather events can have on the marine environment. Excess sediment and resuspension of settled sediment during the event damaged seafloor habitats and ecosystems (Leduc et al, 2024). For example, pre- and post-cyclone imagery data show that populations of kelp, other macroalgae (seaweed) and sponges in the Wairoa Hard seabed area of Hawke’s Bay were almost completely lost following Cyclone Gabrielle. It is highly likely that the loss was due to sediment impacts from the cyclone. Seafloor animals in Hawke’s Bay and Gisborne also showed reduced abundance and diversity post-cyclone. Modelling suggests that recovery time may vary across types of organisms, and continued seabed trawling may slow down recovery time (Leduc et al, 2024).
• Storm-driven run-off can introduce pollutants, nutrients and organic matter into coastal waters. This can lead to harmful algal blooms and oxygen depletion, which threaten marine life and human health. These effects compound existing pressures from urbanisation, agriculture and infrastructure development (see Our environment 2025, section 1).

Land-based activities can degrade the marine environment via sediment and nutrient run-off, and litter

• Run-off and pollutants resulting from human activities on land enter streams and rivers, which then flow into the ocean. Impacts on coastal environments can spread out for kilometres beyond the initial river outflow areas, driven by tides, wind and currents (Jhugroo et al, 2025; Macdonald et al, 2023).
• Human activities on land – such as agriculture, horticulture, forest harvesting and urban expansion – can increase land erosion rates and the amount of fine sediment that reaches marine systems. Excess sedimentation in estuaries and coastal areas can: alter habitats; reduce foraging opportunities for fish like snapper; decrease light; smother sensitive species; and clog the gills of filter feeders such as cockles, pipi and scallops (Booth, 2020; Lowe et al, 2015; Morrison et al, 2009; PMCSA, 2021; Thrush et al, 2021).  
• More frequent and severe extreme weather events due to climate change can exacerbate soil loss, increasing the amount of sediment flowing into coastal areas. For example, significantly higher levels of sediment erosion and deposition were detected in Hawke’s Bay and Gisborne coastal marine areas immediately following Cyclone Gabrielle in 2023 (Leduc et al, 2024).
• Between 2006 and 2020, more coastal and estuarine sites had improving than worsening trends for suspended solids (66 of 84 sites) and for turbidity – a measure of how cloudy the water is – (54 of 87 sites). Monitoring sites were not distributed evenly around the coastline, and were often clustered around urban centres (see indicator: Coastal and estuarine water quality, Our environment 2025, section 4 and Our environment 2025: Technical annex).  
• Land activities such as dairy farming, horticulture and urbanisation can also lead to elevated levels of nutrients such as nitrogen and phosphorus in estuaries and coastal areas. This can result in nutrient enrichment, algal blooms and depleted oxygen, causing harm to plants and animals (Dudley et al, 2020; Plew et al, 2020; Salmond & Wing, 2022). Between 2006 and 2020, more coastal and estuarine sites had improving than worsening trends for nitrogen and phosphorus measures (see indicator: Coastal and estuarine water quality).
• Pathogens such as faecal bacteria can enter estuaries and coastal waters from sources including animal excrement and wastewater discharges, and these can harm ecosystems and people (LAWA, 2023; see Our environment 2025, section 3). Between 2006 and 2020, faecal coliform levels were improving at 50 percent of coastal and estuarine monitoring sites (25 of 50 sites) and worsening at 26 percent (13 of 50 sites) (see indicator: Coastal and estuarine water quality).
• Sea lions (Phocarctos hookeri) and some dolphins are at risk when rainwater and run-off contaminated by cat faeces transport the parasite Toxoplasma gondii to the marine environment. High rates of infection and subsequent mortality from toxoplasmosis in Hector’s (Cephalorhynchus hectori) and Māui (Cephalorhynchus hectori maui) dolphins are the suspected cause of approximately 25 percent of deaths between 2007 and 2018 (Roberts et al, 2019; Roe et al, 2013, 2017). Increased rainfall under a changing climate will likely elevate this risk.
• Litter (including plastics and microplastics) puts marine habitats at risk when it enters the marine environment. In 2023, 67 percent of items counted on beaches in litter surveys were plastic (Litter Intelligence, nd). Seabirds and other marine species are at risk from eating or getting tangled in plastics, which can result in injury or death (Buxton et al, 2013; Clark et al, 2023; PMCSA, 2019). For more information on plastics accumulating throughout the marine environment and the effects on marine species, see Our environment 2025, section 4. 
• For more information on how land-based activities can affect coastal ecosystems, see Our environment 2025, sections 2 and 4, and Our environment 2025: Technical annex.

Commercial and recreational activities affect the marine environment

• Commercial fishing methods in New Zealand include trawling, dredging, potting, longlining and set netting. Trawling is the most common (MPI, nd-d; see Our environment 2025 and Our marine environment 2022). 
• Most scientifically evaluated fish stocks are meeting or exceeding specified performance measures, but some fish stocks continue to be overfished (MPI, nd-e).  
• In 2024, 88 percent of assessed fish stocks in the New Zealand Quota Management System (128 of 146 stocks) were fished within specified limits. Among the 128 stocks where fishing complied with the limits, 105 were fished at or above their management goals (MPI, nd-e).  
• However, 12 percent of assessed stocks (18 of 146) were overfished or depleted in 2024, such as some stocks of black cardinalfish, oysters, orange roughy, scallops and tarakihi. These overfished stocks are being managed so they will rebuild back towards target levels. Six stocks had collapsed (MPI, nd-e).  
• The total estimated catch from recreational fishing in 2022/23 was 3.7 million fish and 1.6 million shellfish. Snapper, kahawai, blue cod and red gurnard together made up 80 percent of all fish harvested by recreational fishers (MPI, nd-b).  
• Recreational fishing declined between 2017/18 and 2022/23 in terms of the number of fish harvested, particularly in the north of New Zealand. Contributing factors include extreme weather events in 2023, prolonged La Niña conditions and changes in fishing habits (MPI, nd-b).

Bycatch continues to contribute to population decline and extinction risk of some protected species

• ‘Bycatch’ refers to non-target species, including protected species, unintentionally captured during fishing (MPI, 2021b). Removing or killing important species through bycatch threatens biodiversity and puts pressure on marine ecosystems (Komoroske & Lewison, 2015).  
• Since 2019/20, reported Hector’s dolphin deaths due to commercial bycatch have ranged from 0 to 5 each year. After the roll-out of cameras in 2023/24, 15 deaths were reported for that year. All 15 of those reported deaths occurred on the east coast of the South Island, where an estimated 9,700 (65 percent) of the total estimated 14,849 Hector’s dolphins reside (DOC, nd; FNZ, 2025; Roberts et al, 2019). Although the 2023/24 figure was above historical reporting levels, it was broadly in line with estimated captures predicted in fisheries risk assessment modelling. The increase is considered to reflect improved reporting of Hector’s dolphin captures following the use of improved technology (FNZ, 2025). 
• Other marine mammals, reptiles, large numbers of seabirds and some protected corals are also caught as commercial bycatch. In 2024/25, 477 fur seals (Arctocephalus forsteri) and sea lions and 2,225 seabirds were reported caught in trawling, longline and set-net fisheries (MPI, nd-f). In the 2022/23 fishing year, 6,704 kilograms of protected coral were reported as bycatch, compared with 2,073 kilograms reported in 2021/22 (McGovern, 2024; McGovern & Hewetson, 2025).  
• Seabirds can be caught in fishing-net mesh or experience fatal interactions with trawl warps (cables). A significant number of seabirds killed by warp strikes may be unobserved, meaning that it is necessary to use cryptic mortality multipliers (a correction factor for estimated deaths based on observations) to more accurately estimate total deaths (Meyer, 2023). Large bycatch events, especially during the breeding season, can lead to chicks starving when foraging adults are caught. 
• Between 2 and 34 captures of protected sea turtles during commercial fishing were reported each fishing year from 2007/08 until 2020/21, when 58 captures were reported (Dunn et al, 2022). In the first three-quarters of the 2024/25 fishing year, 53 sea turtle captures were reported (including 43 leatherback turtles (Dermochelys coriacea)), and all of these sea turtles were released alive (MPI, nd-f). The increase in captures in New Zealand waters may be occurring because warming sea-surface temperatures are bringing sea turtles further south.

The area trawled each year has been declining, but trawling still affects the seafloor and the animals that live there

• Commercial fish trawling and dredging have lasting impacts on the seabed and its habitats. Effects include altering seabed structure, damaging habitats and reducing marine populations (Clark et al, 2019, 2022a; MPI, nd-d).
• Between 1990 and 2024, about 11 percent of New Zealand’s exclusive economic zone and territorial sea was trawled. The area trawled each year has been declining over time. In 2023/24, 68,048 square kilometres were trawled, which is a decrease of 7.5 percent from 73,567 square kilometres in 2022/23 (MPI, nd-d). Recent years have seen a reduction in bottom-contact trawling by both deepwater and inshore fisheries (MacGibbon et al, 2024).
• Underwater seamounts, knolls and hills can be highly productive and support rich biodiversity, including high densities of protected habitat-forming corals. They are also targeted by some deepwater fisheries (Clark et al, 2019, 2022b). Eleven percent of seamounts, knolls and hills in New Zealand’s exclusive economic zone and territorial sea were trawled at least once between 1989 and 2019 (Clark et al, 2022b).
• Recovery of long-lived, slow-growing animals impacted by trawling, such as corals, is slow. For example, it took about two decades for the first signs of coral recovery to appear after trawling ceased on the Graveyard Knolls of the Chatham Rise (Clark et al, 2022a).

Aquaculture can have both positive and negative effects on marine ecosystems

• The growing aquaculture industry, which includes species like green-lipped mussels, Chinook salmon and Pacific oysters, can have both positive and negative impacts on marine ecosystems (Howarth & Major, 2023; MPI, 2013).
• Mussel farming, for example, can support seafloor communities and some wild fish. However, it can lead to local enrichment of the seabed and alter the composition of sediments (Howarth & Major, 2023; Underwood et al, 2023). Farm infrastructure can also disrupt currents, damage the seabed, shade the seafloor and put wildlife at risk of entanglement (Howarth & Major, 2023; MPI, 2013).

Many indigenous marine species are threatened or at risk

• The pressures from land-based and ocean-based activities outlined above combine to impact indigenous marine species. These pressures are often compounded by climate change, which directly impacts indigenous species through its effects on the temperature, acidity and circulation of global oceans.  
• In 2021, 91 percent (82 of 90) of indigenous seabird species were threatened with extinction or at risk of becoming threatened, including 16 species identified as taonga. Estimated population trends show 27 percent of species have decreasing populations, while 18 percent are increasing, 12 percent are stable or increasing, and 43 percent are stable (see indicator: Extinction threat to indigenous species).
• In 2024, 35 percent (14 of 40) of indigenous marine mammal species were threatened with extinction or at risk of becoming threatened. Estimated population trends show 3 percent have increasing populations, 15 percent of species have decreasing populations, 27 percent have stable populations and 55 percent have no estimated trend (Lundquist et al, 2025; see Our environment 2025: Technical annex).  
• In 2016, 9 percent (10 of 107) of shark, ray and chimaera species were threatened with extinction or at risk of becoming threatened, including one species identified as taonga. Estimated population trends show 2 percent of species have increasing populations, 1 percent have decreasing populations and 54 percent have no estimated trend (see indicator: Extinction threat to indigenous species and Our environment 2025: Technical annex). Since the introduction of the 2008 New Zealand threat classification system, the only marine fishes assessed have been sharks, rays and chimaeras (see Our marine environment 2022).
• In 2021, 57 percent (449 of 786) of assessed indigenous marine invertebrate species were threatened with extinction or at risk of becoming threatened. Estimated population trends show 5 percent of assessed species have decreasing populations, 66 percent are stable and one species is increasing¹ (Funnell et al, 2023; see Our environment 2025: Technical annex).

¹ Only some known species of marine invertebrates have been assessed for extinction threat status and estimated population trends.

Climate change exacerbates risks from invasive species and diseases

• Climate change increases the chance that established pests will spread further, reproduce faster and have more severe adverse impacts on biodiversity (Bollen et al, 2016; Spyksma et al, 2024). Higher water temperatures may also increase the risk of new invasive pests and diseases becoming established (Keegan et al, 2022; Rowley et al, 2024; Wesselmann et al, 2024).  
• The occurrence of tropical fishes in temperate regions can be an indicator of climate change impacts. Tropical, subtropical and rare fishes have been documented in New Zealand’s waters for more than a century, but the occurrence and diversity of warmer-water species have increased over the past 50 years. This may signal a climatemediated shift in New Zealand’s marine biodiversity (Middleton et al, 2023). 
• Marine heatwaves can facilitate the spread of some invasive species. For example, the cover and abundance of sea squirt (Symplegma brakenhielmi) was observed to rapidly increase during a marine heatwave (which included growing over other invertebrate and seaweed species), and to decrease (but not disappear) as water temperatures reduced (Spyksma et al, 2024).  
• The interactions between parasites and hosts can change due to global and local stressors, including warming waters and ocean acidification associated with climate change and marine pollution. An increase or decrease in disease can have effects at individual, population and community scales (Lane et al, 2022).

More non-native species are in our marine waters and are spreading to new locations

• Non-native marine species are being introduced continually to New Zealand waters, usually carried by ballast water or on the hulls of shipping and recreational vessels (Davis & Hepburn, 2020; see indicator: Marine non-indigenous species: Data to 2022).
• As of 2022, a total of 428 non-native marine species have been found in marine waters around New Zealand; 62 percent (266) of these have established populations here. Between 2010 and 2022, 73 new non-native species were found, 44 of which have become established (see indicator: Marine non-indigenous species: Data to 2022).
• Certain invasive species can change how coastal marine seafloor ecosystems function. The Mediterranean tubeworm (Sabella spallanzanii) is one example. Its behaviours of filter feeding and building tubes from sediment grains (eg, sand and shell fragments) may alter the flow of sediments and what they are made up of. This affects how nutrients are recycled, as well as how organic matter is distributed through the ecosystem (Tait et al, 2023).
• Non-native marine species can spread rapidly through New Zealand’s marine environment. For example, the invasive crab Charybdis japonica has increased in distribution and abundance around the North Island since it was first detected in Waitematā Harbour in 2000. It is now found in Tauranga and Ōhiwa harbours, 200 kilometres further south than previously reported. It has also spread northwards as far as the Bay of Islands and to the west coast, in the Hokianga, Kaipara and Manukau harbours (Hilliam & Tuck, 2023).
• Two new Caulerpa seaweed species were recorded at Aotea Great Barrier Island in 2021 (see indicator: Marine non-indigenous species: Data to 2022). Globally, many Caulerpa species are considered highly invasive, having impacts on fish, invertebrates, native seaweeds and seagrass and affecting nutrient cycling. Preliminary New Zealand data suggested Caulerpa may have negatively affected two taonga species: tipa (scallops, Pecten novaezelandiae) and kina (sea urchins, Evechinus chloroticus) (Middleton, 2023). By August 2024, Caulerpa had spread to more than 1,500 hectares of the upper North Island seabed, competing with other species, disrupting local ecosystems and posing risks to recreational, cultural and commercial marine activities (MPI, nd-c).
• Existing and emerging bacteria and associated diseases have been identified as factors in the failure of toheroa (Paphies ventricosa) populations to recover from overfishing (Bennion et al, 2022) and skin infections in fish (Rudenko et al, 2025). Oysters in New Zealand are under threat from the parasite Bonamia ostreae and the ostreid herpesvirus type 1 (OsHV1), both of which can cause oyster death. OsHV-1 may be transferred to new locations and species on the hulls of vessels (Fuhrmann et al, 2023).

Climate change is impacting many species and habitats already under pressure from other human activities

• Marine heatwave impacts have been documented (and predicted) for sponges, kelps, reef communities, fish, whales, turtles and penguins in New Zealand (Barlow et al, 2023; Behrens et al, 2025; Bell et al, 2023, 2024; Dunn et al, 2023; Montie & Thomsen 2023; Salinger et al, 2023).
  
• In years when sea-surface temperatures are warmer than usual, there is reduced survival of adult yellow-eyed penguins (Mattern et al, 2017). Marine heatwaves have contributed to local extinctions in southern bull kelp and may be linked to starvation of little penguins (Salinger et al, 2020). Marine heatwaves have led to severe bleaching and necrosis in sponges and a rapid expansion of invasive sea squirts (Bell et al, 2023, 2024; Salinger et al, 2023; Spyksma et al, 2024). Commercial fisheries and aquaculture have also been affected (see section 3).
  
• Impacts on one species can ripple across an entire ecosystem (Montie & Thomsen, 2023). For example, some areas where bull kelp (Durvillaea spp.) was completely lost during the 2017/18 heatwave were colonised by an invasive, non-native kelp (Undaria pinnatifida). This coincided with a decline in green-lipped mussels (kuku/kūtai) – an important mahinga kai (traditional food-gathering) species (Awatere et al, 2021; Thomsen et al, 2019).
  
• Warming ocean temperatures in northern New Zealand have coincided with areas of increased abundance of long-spined sea urchins – an indigenous species known to have large impacts on kelp forests (Balemi & Shears, 2023). Urchin barrens are areas where kelp forests are stripped due to overgrazing by sea urchins, often in the absence of predators like snapper and rock lobster. This results in reduced biodiversity, altered ecosystem structure and diminished habitat for fish and invertebrates (Balemi & Shears, 2023; Kerr et al, 2024). There is a risk that urchin barrens could expand with further spread of long-spined sea urchins, in association with ocean warming (Kerr et al, 2024).
  
• Warmer temperatures and ocean acidification are expected to make it harder for species such as molluscs and corals to grow and maintain their shells and skeletons (Anderson et al, 2022; Böök et al, 2024; Law et al, 2018; McCullough et al, 2024).
  
• Exposure to changing ocean temperatures and acidification can exacerbate the negative impacts of other stressors that affect the survival of marine species. For example, in response to high temperature and low pH, the mottled brittle star (Ophionereis fasciata) – which is only found in New Zealand – demonstrates altered respiration, regeneration and growth, and survival (Márquez-Borrás & Sewell, 2024).

Coastal habitats protect against sea-level rise and coastal hazards, but many have been degraded

• Coastal habitats – including beaches, dunes, wetlands (eg, saltmarshes and mangroves), seagrasses, seaweed forests and shellfish reefs – provide a range of functions and services (Geange et al, 2019). These habitats play a role in stabilising sediments, recycling nutrients and supporting biodiversity, and they also provide great cultural, recreational and economic value (Anderson et al, 2019; Bulmer et al, 2024c; see Our environment 2025).
• Many of these habitats are found in estuaries, where seawater mixes with freshwater. Estuarine habitats support unique vegetation, invertebrates, fish, shellfish and bird species – enhancing biodiversity and supporting fisheries and ecosystem productivity (Rullens et al, 2022).
• These habitats buffer coastlines from hazards such as flooding and erosion. For example, mangroves have root systems that anchor soil and reduce coastal erosion, buffering coastlines from coastal and river flooding. They also trap sediments and filter pollutants from land-based sources (Shah & Ramesh, 2022).
• Alongside other benefits, native coastal vegetation, wetlands and dunes can help protect coastal ecosystems from excess sedimentation, nutrient pollution and coastal flooding (Allan et al, 2023; Thompson, 2022). Sand-trapping vegetation can also help increase the resilience of coastlines to erosion (Coastal Change, nd).
• Open coast habitats such as kelp forests are capable of dampening wave energy, which can reduce coastal erosion and sedimentation (Steneck et al, 2002).
• Despite their importance, many of these habitats are under pressure from coastal activities and development, altered freshwater inputs and excessive land-based contaminants, including sediments, nutrients and plastics (see Our environment 2025).
• Most coastal habitats have experienced loss or damage, with some exceptions such as mangroves (Anderson et al, 2019; Bennion et al, 2024; Morrison et al, 2014a; Suyadi et al, 2019).
• Between the 1950s and 2008, active sand dune extent decreased around 80 percent (see indicator: Active sand dune extent). Remaining active dunes are increasingly threatened by introduced marram grass. Coupled with livestock grazing, land development, the effects of vehicles and erosion, and coastal squeeze, the expansion of marram grass degrades the integrity and function of dune ecosystems (Dune Restoration Trust of New Zealand, 2014; Thompson, 2022; see Our land 2024).

Coastal habitats store carbon, but their loss or degradation can release it back into the atmosphere

• The ocean holds the second-largest amount of carbon in the Earth system, second only to terrestrial carbon storage, and approximately 60 times more than the atmosphere (DeVries, 2022). ‘Blue carbon’ is the term used for carbon stored in ocean and coastal ecosystems.
• The seafloor in New Zealand’s exclusive economic zone is estimated to contain about 1 percent of the global seabed organic carbon store (Nodder et al, 2023). An inventory of New Zealand marine carbon concluded that data are insufficient to reliably describe the marine carbon budget (accounting for how carbon is stored in, moves through and is released from marine systems) (Nodder et al, 2025).
• Coastal blue carbon refers to rooted vegetation in the coastal zone, such as seagrass meadows, mangroves and tidal marshes. These coastal ecosystems have high rates of carbon sequestration (10 to 45 gigatonnes of carbon globally) compared with terrestrial habitats, but store less carbon than the open ocean (37,000 gigatonnes in the form of dissolved inorganic carbon globally) (Friedlingstein et al, 2025; Friess et al, 2024; Warnell et al, 2022).
• Coastal blue carbon systems have potential to contribute to offsetting carbon emissions and climate change mitigation if they are protected or restored (Howard et al, 2023). These coastal ecosystems store carbon primarily in soils and sediments, where it can remain for long periods (Bulmer et al, 2024c). Protecting and restoring these habitats also supports biodiversity and ecological health.
• The types of coastal blue carbon habitat vary by region, as does land availability for restoration opportunities and potential carbon market benefits. The first national assessment of blue carbon habitats in New Zealand estimates estuaries and coastal areas contain 20,932 hectares of saltmarsh, 30,533 hectares of mangroves and 61,340 hectares of seagrass, which are estimated to collectively sequester 57,800 tonnes of carbon a year (Bulmer et al, 2024c).
• Unvegetated estuarine habitats, such as those dominated by shellfish and microscopic algae, also store significant amounts of carbon due to their large area, sequestering an estimated 164,483 tonnes of carbon a year in New Zealand (Bulmer et al, 2024c). There is potential to restore a further 56,482 hectares of saltmarsh, 17,291 hectares of mangroves and 14,087 hectares of seagrass. If all potential areas were restored, New Zealand would achieve a total sequestration potential of an estimated 91,680 tonnes of carbon a year (Bulmer et al, 2024c).
• Mangrove forests have expanded in many areas over at least the past 50 years, due to increased sedimentation (Anderson et al, 2019; Jones et al, 2022; Morrisey et al, 2010). For example, Auckland’s mangrove area has increased an average of 3.2 percent annually from 1940 to 2014 (Suyadi et al, 2019).
• In New Zealand, seagrass area decreased during the past century, particularly in highly impacted harbours, where records showed reductions ranging from 40 percent (at Whangamatā Harbour) to 90 percent (eastern Bay of Islands) (Morrison et al, 2014b). However, seagrass meadows have shown signs of recovery in some locations in recent decades. In the Waitematā Harbour, seagrass has increased exponentially since 2004 (Lundquist et al, 2018), and in Whangārei Harbour transplanting has been found to be an effective method for rehabilitation (Matheson et al, 2017). These systems still face threats from sea-level rise, rising temperatures, nutrient pollution, increased turbidity and disease (Turner & Schwarz, 2006).
• Salt marshes and other saline wetlands have shown small changes in extent over the past 20 years (Bulmer et al, 2024c). An estimated 90 percent of wetlands have been lost since pre-European settlement (Dymond et al, 2021). Between 1996 and 2018, saline wetland areas decreased by 180 hectares (see indicator: Wetland area).
• Degraded habitats have lower carbon sequestration (Thomson et al, 2025). Loss and degradation of coastal blue carbon ecosystems result in the release of large quantities of stored carbon back into the atmosphere (Bulmer et al, 2024c). Our remaining wetlands continue to degrade due to drainage, pollution, increased sedimentation, invasive weeds, animal pests and climate change (see Our land 2024).
• Bottom trawling can re-suspend carbon from the seabed into the water column, although the amount re-suspended is currently uncertain. This carbon may eventually be released into the atmosphere, contributing to greenhouse gas emissions. Areas such as Fiordland, the Chatham Rise and the Bounty Plateau are particularly rich in carbon but also vulnerable to disturbance (Nodder et al, 2023). 

Rising seas and infrastructure limit the natural movement of coastal habitats

• In response to rising seas, coastal habitats such as dunes and wetlands would typically migrate landward. However, much of New Zealand’s coastline features natural barriers such as cliffs, as well as built infrastructure such as seawalls, which protect our homes and communities from rising sea levels. Pasture and exotic vegetation can also act as barriers. These barriers block the movement of coastal habitats, leading to a phenomenon known as ‘coastal squeeze’.
• Coastal squeeze may cause degradation and loss of important coastal habitats. It may also reduce the natural protection that these habitats can provide (Allan et al, 2023; Davis-Jones, 2025; Douglas et al, 2022; Stewart et al, 2020).
• Coastal squeeze will alter the distribution of intertidal habitats and the animals and plants within them, as well as altering coastal ecosystem functions and services (Rullens et al, 2022).
• In New Zealand, coastal hardening (building engineered structures such as seawalls for coastal protection) is expected to increase by 49 to 76 percent over 25 years, starting from 2018 (Floerl et al, 2021). Such structures will limit the ability of intertidal areas to shift landward with sea-level rise, resulting in coastal squeeze – and, in turn, leading to degradation and ultimately loss of habitat (Mangan et al, 2020).