Ocean temperatures in Hawke's Bay are up to 6 degrees Celsius higher than normal, with scientists warning the scorching seas could have "catastrophic" impacts on marine life.
Niwa experts say if sea temperatures, which are almost 24C north of Napier, remain high for another two weeks it would likely be classified as a marine heatwave.
Niwa climate specialist Ben Noll said the average temperature at this time of year should be nearer to the 18C-20C mark.
It was the second year in a row New Zealand had dealt with "pretty extreme conditions", he said. Temperatures in the Tasman Sea last year were up to 6C higher than normal.
While it might be nice for your summer dip, the warming waters - fuelled by climate change - could permanently hurt our marine ecosystem, University of Auckland marine scientist Dr Andrew Jeffs said.
Last summer's marine heatwave caused a high mortality rate in salmon in the Marlborough Sounds, with millions of dollars in losses. In the Hauraki Gulf temperatures as high as 25C caused issues for the green-lip mussel farming industry.
"They were simply falling off the lines," Jeffs said.
"Sea temperatures do naturally fluctuate, but very gradually and only a small amount. Most species only have a narrow range they can survive in."
When sea temperatures change, species generally moved within their range, further south or into deeper water.
"But when they can't move, eventually we start to lose species."
Marine organisms under stress are also more susceptible to disease.
"It basically puts the whole ecosystem under stress."
Warmer seas could also attract new predators and competition for local species.
Despite the rise in sea temperatures, Hawke's Bay Regional Council water quality and ecology scientist Dr Gary Rushworth said there had so far been no incidents such as fish deaths or algal blooms and none reported by members of the public.
The warmest sea temperatures were situated from the Hastings region up to the Wairoa Border, he said.
"We have maps where we can see specks of temperatures ranging from 23C to 23.5C just north of Napier."
However, beaches south of Cape Kidnappers were sheltered from it.
"Ocean Beach is sitting between 18C to 19C, so as we get further south it gets cooler."
Noll said temperatures started warming up in December but they started to intensify in January.
"It's basically on par with what we saw last year which was extremely unusual. It will be interesting to see if it persists."
Noll said a cool front was approaching the country next week which might hinder the heatwave or stop it in its tracks.
Fishermen aren't complaining about the warmer sea temperatures, saying it could attract new species to the local waters.
MegaFish competition convener Jo Bicknell said fisherman might start to notice a difference within the next couple of weeks.
"The bigger fish prefer warmer water and follow it down from the tropics."
A recent catch of a mahi-mahi in Hawke's Bay waters was "quite unusual", he said.
"I'm expecting it to be a good season."