The government study found New Zealand's oceans are growing hotter, turning more acidic and already threatening thousands of coastal homes as sea levels rise.
The waters lapping New Zealand are now warming 34 percent faster than global averages, according to the Our Marine Environment report.
"Climate change is driving significant changes in our oceans," the report said.
"Ocean temperatures are increasing, and marine heatwaves are becoming more frequent, intense and longer-lasting.
"Sea-level rise is accelerating at many locations."
Researchers have linked global warming to disruptions in the vast oceanic currents that swirl between New Zealand and Antarctica.
New Zealand's proximity to these currents -- as well as other changes in atmospheric circulation -- helped to explain why these seas were warming so quickly.
Department of Conservation marine advisor Shane Geange said the report showed how "climate change is making existing problems in our oceans even worse".
"For example, global warming has made New Zealand's sea temperatures rise much faster than the worldwide average.
"This means we're seeing more frequent and severe marine heatwaves. Sea levels (are) rising more quickly, and our oceans are becoming more acidic and losing oxygen."
Sea-surface temperatures at four sites around New Zealand rose, on average, between 0.16C and 0.26C per decade between 1982 and 2023.
Many native species were struggling to adapt to life in warmer and more acidic oceans, the study found, and risked being overrun by invasive pests.
Warmer-than-normal sea temperatures have previously been linked to deaths of New Zealand's native yellow-eyed penguins.
"Because of these findings, how we manage New Zealand's marine environment needs to change," said Geange.
"We now have enough evidence to take action, and delaying risks further harms to our marine ecosystems."
Steadily rising sea levels were already being felt in many of New Zealand's low-lying coastal hamlets, the report found.
More than 200,000 homes worth US$100 billion (NZ$180 billion) were found in areas at risk of coastal inundation and inland flooding.
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