New international research shows marine heatwaves are becoming more common and likely to get much stronger.
Marine heatwaves are smaller and more rapid changes in sea temperature that occur during hot weather. New Zealand experienced one last summer.
The research, published in Nature Climate Change, shows the heatwaves are 54 percent more common internationally than they were 75 years ago.
And they are likely to become more frequent and hurt marine ecosystems.
One of the researchers, Dr Mads Thomsen from the University of Canterbury, said marine heatwaves were first recorded in New Zealand during last year's record-breaking summer.
Dr Thomsen said more research was needed to understand their impact on marine ecosystems here.
Early evidence suggested rapid changes to local marine systems, including plankton, fish and kelp, he said.
"Marine ecosystems face many threats, such as invasions by non-native species, overfishing, acidification and pollution, but shorter periods of unusually high temperatures is now also a well-documented cause of rapid biological changes, sometimes leading to loss of habitat, reduced fisheries or even local extinctions."
In general, New Zealand waters are getting warmer.
Niwa physical oceanographer Phil Sutton was the lead author on a study tracking ocean temperatures around New Zealand over the past 36 years.
"Since 1981 we are talking of warming of about 0.1 to 0.3 degrees Celsius per decade. That may not sound like a huge amount but slightly stronger warming of about 0.4C per decade off the east coast of Tasmania has resulted in significant changes to ecosystems which has led to concerns of similar impacts in New Zealand," Dr Sutton said.
"The 30-year time series is long enough to see long-term changes and there is no reason to think this will turn around."
The strongest warming occurred off the Wairarapa coast, while the weakest was along the north east coast between the east and north capes.
The increased warmth penetrated in places to 200m deep while in the Tasman Sea it reaches down to at least 850m.
The most obvious cause? Climate change, Dr Sutton said.
It's not just happening here, either.
The world's oceans are warming about 40 percent more quickly than previously thought.
Dr Victoria Metcalf works in the Office of the Prime Minister's Chief Science Advisor and said warmer seas could result in undesirable species coming to New Zealand waters.
"When they come down into an ecosystem which they previously haven't been a member of they can upset the balance of that ecosystem, so you may see you know, predatory activity occurring.
"There could be a new sea urchin species that ends up here and contributes to the kina barrens and the devastation of the kelp forests, that sort of thing."
Warming seas could effect "every species", she said.
"You could see a shift in plankton blooms, plankton being the primary producers at the bottom of the food chain and again, I think from the satellite data we are already seeing some shifts in that … you muck with the primary producers and you muck with everything else in the ecosystem."
It wouldn't take long to have an impact.
"Small shifts can - pretty much within a season but definitely within a decade - result in significant changes."
But there would be winners and losers.
"Some species actually benefit from a change in conditions. So it's not a one size fits all response."