And as southern sea temperatures continue to rise, questions have emerged about the implications for marine life.
Stats NZ yesterday released data showing between 2022 and last year, oceanic and coastal waters around New Zealand reached their warmest annual temperatures since the data series began in 1982.
Environment and agricultural statistics senior manager Stuart Jones said each oceanic and coastal region around the country experienced their hottest years on record in either 2022 or last year.
Coastal regions warmed on average by 0.19°C to 0.34°C per decade and the east coast of the South Island led the way.
"Even small rises in temperature can disrupt marine ecosystems, cause some species to relocate, and increase disease risks," he said.
"It also contributes to sea-level rise as the warmer water expands."
MetService Research and Innovation Hub general manager Dr Brett Beamsley said the data Stats NZ released clearly showed surface temperatures increasing, but the "whole volume" of the ocean was getting warmer.
The sea surface was simply the easiest thing to measure.
"The whole volume is increasing in terms of temperature and that’s having an impact on marine ecosystems."
Dr Beamsley said he was not surprised the statistics showed the east coast of the South Island as having the largest average increase.
Questions had emerged of late about the Southland current — a large oceanic current that comes up around Stewart Island and up past Otago and the southeast coast of New Zealand.
It could be the water within that current was becoming warmer, or it could be the current had shifted further offshore, which would allow more warm coastal waters to upwell.
Whatever went on in the atmosphere would impact the oceans and whatever happened in the ocean would impact the atmosphere, he said.
"We can see the linkage between extreme events in the atmosphere correlated with warmer oceans — you could have larger rainfall events, for example.
"As we’ve seen these changes in the oceans, we’re going to see the changes equally in the land in terms of extreme events.
"It’s that kind of linked system that we need to understand a little more about and what the impacts are."
University of Otago climate and sea ice researcher Dr Andrew Pauling said the Southland current appeared to have changed in the last year, but the reasons for the change, or if it was simply an anomaly, needed to be better understood.
A change would have important implications for the local ocean and life that lived there.
University of Otago marine ecologist Dr Rebecca McLeod said marine heatwaves — sea temperature rises of more than 5°C on average — could "really test the tolerance of a species" in the short term.
However, the slower, average increases appeared to bring about a shift in the distribution of mobile species.
In Fiordland, species that could not get out of the way, such as sea sponges, had died or showed signs of stress in marine heatwaves.
Tuna, for example, were becoming more of a feature in the area where it was now common to see them in big schools. They were "really voracious predators" and the impact they would have on other species that lived there remained to be seen.
