Our blue backyard is being squeezed more than ever, a major new stocktake shows, as another severe marine heatwave looms for New Zealand.

The Government's latest report on the state of our marine environment bears little good news, with ocean pollution, fishing bycatch and a fast-worsening climate crisis all putting ecosystems under increasing pressure.

New Zealand's Exclusive Economic Zone is 20 times larger than its land mass and supports a blue economy estimated to be worth $4 billion a year.

Around 80 per cent of New Zealand's flora and fauna can also be found in its 4.4 million sq km marine estate - yet scientists have only identified less than one quarter of the species they believe live there.

"When the marine environment is under stress, it can affect ecosystems and things that are important to New Zealanders," said Natasha Lewis of the Ministry for the Environment, which publishes the three-yearly stocktakes with StatsNZ.

"For example, climate change is causing sea level rise, which impacts coastal communities, sites of cultural and ecological significance, and marine species."

The report noted how ocean acidity in waters south of the country has increased by nearly 9 per cent in just over 20 years, while sea surface temperatures have been rising at a rate of 0.1 to 0.2C per decade – and at a quicker rate in our coastal waters.

Persistent marine heatwave conditions have been causing damaging and widespread impacts for ocean life, while driving hotter temperatures and ongoing glacier melt – and an event forming up this spring could be among the worst yet observed.

Swings in sea temperatures were predicted to drive more intense and frequent marine heatwaves under climate change, with Niwa recently warning that, under one modelled scenario, parts of the country could be experiencing these conditions year-round by the century's end.

A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University

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The rate of coastal sea level rise has also doubled over the past 60 years.

Just 60cm of rise - which may become reality for many locations within decades – was enough to threaten tens of billions of dollars' worth of buildings.

While extreme wave events appear to have been increasing east and south of the country, but becoming less frequent in some other places, it wasn't clear whether these recent trends were part of a long-term trend, or natural climate cycles.

Elsewhere, the report found humans were having more of a direct impact through the pollution we were sending into our waters.

Some pollutants, like pharmaceuticals and cleaning products, were winding up in the marine environment – and scientists were still trying to fully understand their impact.

Plastic, accounting for about 70 per cent of coastal litter last year, was continuing to be found on beaches, throughout the ocean, and inside shellfish, fish, and birds.

It took centuries to break down, and there was increasing evidence to show millimetre-long microplastics was now widespread in our waters.

Inter-tidal sedimentation rates had generally increased and became highly variable since European settlement.

Sediment - made up of fine particles like silt, mud, and organic material that got carried in water – was being washed from pastures, forests after felling, and urban development sites.

It filled in the spaces used by fish and invertebrates for hiding and breeding, making their food harder to find or to eat.

But there were some positives among the latest monitoring data from around the country.

Of indicators influenced by suspended sediment, there were more sites with improving trends than worsening trends for suspended solids and turbidity - a measure of how cloudy the water was.

The general picture was similar for measured nutrients – specifically, forms of nitrogen and phosphorus.

Yet, sites also had more worsening trends than improving ones for visual clarity, dissolved oxygen, and Enterococci – a gut bacteria indicating faecal contamination.

The report also singled out the impact of fishing, finding bycatch of fish and seabirds remained a "significant pressure" on some species.

In 2017, for instance, it was estimated that the non-target catches of invertebrates and fish in offshore fisheries was around 65,000 tonnes per year.

Estimates of marine species themselves, meanwhile, carried some grim figures.

Some 90 per cent of indigenous seabirds, 82 per cent of indigenous shorebirds, 22 per cent of marine mammal species, and 81 per cent of assessed marine invertebrate species were now classified as either threatened with extinction, or at risk of becoming so.

Maui are the world's smallest and rarest marine dolphins, with between 48 and 64 individuals older than a year left. Photo / Supplied

Nearly 30 of 112 identified marine taonga species were also threatened with extinction, with another 57 placed at risk.

For many Māori, the dismal insights laid out in the update came with myriad, profound consequences.

Rising seas and coastal inundation threatened marae, urupā, and wāhi tapu, while high nutrient levels could increase algal blooms – harming kaimoana species and affecting mahinga kai practices, such as harvesting and protecting seafood.

Overall, Niwa marine ecologist Dr Carolyn Lundquist said, the report showed most of the negative trends highlighted in earlier stocktakes had only continued.

"The report shows the connections between our oceans and people, culture and wellbeing, and that these negative trends in ocean health affect not just our native biodiversity, but also our economy, coastal infrastructure and the connections we have with the ocean through recreational use, gathering kaimoana and other tikanga practices."

Niwa scientist Professor Craig Stevens said those indicators that showed stable or improving trends were "intrinsically positive", but also pointed to the value of actually having data - and having researchers who could interpret it.

As the report itself repeatedly pointed out, Stevens said there remained a "severe lack" of marine data for the seas around New Zealand.

"The unknowns are large, especially around conditions and change beneath the ocean surface away from satellite measurement techniques.

"The central role of climate in this report, in conjunction with the very long response times of the marine environment, means that the faster we reduce our emissions, the better the ultimate outcomes."