More extreme marine heatwaves forecast for South Island

More extreme marine heatwaves are forecast for the South Island, as sea surface temperatures continue to rise near Stewart Island and Otago.

MetService oceanographer Dr Joao de Souza said for the second time this year, extreme marine heatwaves were forming in Aotearoa's southern seas.

While there had been almost uninterrupted marine heatwaves for a couple of years in New Zealand waters, the warming had reached new levels, de Souza said.

A marine heatwave is classified as temperatures that are higher than 90 percent of what was seen in the past.

For Stewart Island at this time of year, normal sea surface temperature is about 14.3 degrees Celsius and the marine heatwave threshold is 15.3C.

When tracking marine heatwaves, MetService compares the current or forecasted temperature to the average of the past 25 years for each location and day of the year.

Extreme marine heatwaves are when waters are warmer than four times the difference between normal conditions and the marine heatwave threshold.

"For Stewart Island right now, extreme marine heatwave conditions are reached when the water temperature reaches 18.3 degrees. We are seeing steady temperatures above 18 degrees forecast for this weekend."

De Souza said extreme marine heatwave was the highest category, which reflected how unusual this level of warming was.

"For the second time this year, we are now seeing extreme marine heatwaves building in New Zealand waters, this time near Stewart Island and Otago."

The heatwave surrounding Stewart Island was concerning, because 4-6 degree warming was forecasted to last three or four days, de Souza said.

Christchurch feels the heat

While much of the North Island is still drying out, Christchurch hit its hottest day of the year today at nearly 34C.