The west coast of the North Island is experiencing a "severe marine heatwave" with water temperatures up to a whopping 4C warmer than normal for this time of year.
MetService's oceanographic division MetOcean Solutions warns sea surface temperatures from Northland to Taranaki will rise higher than 22C, which is above what is normal even in the peak of summer.
MetOcean says the entire east coast of the North Island has been experiencing marine heatwave conditions for at least the last two weeks, with strong marine heatwaves occurring in Cape Reinga, the Hauraki Gulf, and the Bay of Plenty.
It's also predicting a marine heatwave in the Chatham Islands as a sharp increase in sea surface temperatures from December 9 puts the region under a strong marine heatwave. These conditions will likely continue for the next six days.
"The forecast for Friday and Saturday is dominated by strong marine heatwaves across the North Island of New Zealand - we rarely see such a large area affected by temperatures 3-5C above average," says Moana Project Science lead Dr Joao Souza.
And there's a warning it could be dangerous for sea life.
"Marine heatwaves may please beachgoers but continued high temperatures can harm marine organisms," Dr Souza says.
"As part of the Moana Project, we are examining the warming at depth, and providing this information to New Zealand's fisheries and aquaculture sectors to help them manage their marine-based businesses."
The cause is a La Niña summer which brings warmer sea temperatures as well as warmer and more humid air - meaning it's hotter for both sea and land.
"The extra cloud means overnight temperatures can be brutal for some, with lows in the upper North Island not dropping below 20C for over the next week in some northern areas," WeatherWatch says.
"The layer of humidity means the Humidex (feels like) reading may actually be well into the mid-20s at sunrise."
However there is good news for vulnerable sea life: MetOcean's marine heatwave forecast shows the warm water dissipating at the beginning of next week.
"Although marine heatwave conditions may last longer, the temperatures will become more moderate," Dr Souza says.
"The short-term nature of the warming indicates that it is likely caused by warm air heating the sea surface, rather than warm water moving south."