NIWA forecasters say a marine heatwave has formed around northern Auckland and Northland after a big increase in sea surface temperatures (SSTs) last month.
Coastal temperatures in these areas are 1.6C above the November monthly average and are now running between 18 to 21C.
"We're not even at the peak of our sea surface temperatures which typically occur over January and February," says NIWA meteorologist Ben Noll.
And there is a medium-to-high chance for a marine heatwave to develop in other regions over the next several weeks, with sea surface temperatures in other areas between 0.7 to 1.1C above average.
What's causing the marine heatwave?
NIWA says there are several factors including La Niña's northerly winds, pressure patterns, and climate change.
"High pressure systems in October brought more sunshine, warmer temperatures and less wind than normal. This pattern led to warming of the sea surface and prevented cooler water underneath from mixing to the top," Noll says.
"While the weather has been a bit more unsettled during early November, ocean temperatures have remained warmer than average due to prevailing northeasterly and northwesterly air flows."
Noll says New Zealand is now being affected by a La Niña climate regime. La Niña is a complex weather system caused by strong wind blowing warm water at the ocean's surface from South America and across the Pacific Ocean.
This tends to bring more northerly winds and has historically been associated with warm Tasman Sea temperatures.
The increase in global warming means more heat is absorbed by the sea.
A report earlier this year found the amount of energy that's warmed the world's oceans over the past 25 years is the same as if 3.6 billion atom-bombs were dropped into them.
"The oceans are a repository of the excess heat that is caused by global warming, and represent one of the best ways that we can measure climate change anywhere on the globe," Noll told Newshub at the time.
What impact will a marine heatwave have on New Zealand?
NIWA says one of the main reasons New Zealand had such a hot start in 2019 was the presence of above-average ocean temperatures.
"While the next week will have some variability across New Zealand, air temperatures are most likely to lean in a warmer direction overall," Noll says.
"Some particularly warm temperatures are possible to end the month."
Warmer-than-average seas can also provide extra energy for passing storms, and can also impact on our fisheries.