An unprecedented marine heatwave is causing Tasman sea temperatures to peak at up to six degrees above average.

Photo: rafaelbenari/123RF

Since November, the water has been more than 2°C above average, peaking even higher on some days.

"It's never been that hot before," climate scientist Jim Salinger said.

"We looked at records back to 1900 and there's nothing anywhere near this."

Fishers in Doubtful Sound and Fiordland had reported catching snapper for the first time, while there are also anecdotes from surf lifesavers of bluebottle jellyfish appearing much earlier than normal, Dr Salinger said.

The balmy ocean temperatures were being driven by a combination of two different climate patterns: the southern annular mode, and a La Nina weather system, he said.

The southern annular mode was in its positive phase, meaning westerly winds that swirled around the Southern Ocean had contracted towards Antarctica.

"We've not had the usual incursions ... of strong westerly winds, so that's really shut the door from the Southern Ocean for outbursts of cold air and rough weather to stir up the Tasman Sea."

The La Nina pattern meant more north-easterly winds, which encouraged the warm east Australian current to come further south towards New Zealand, Dr Salinger said.

Readings from floating gauges monitored by NIWA showed the warmer water was up to 50m deep, he said.

Our Changing World host Alison Ballance said extreme systems like the heatwave could have a profound effect on marine ecosystems.

Kaikōura biologist Jim Mills, who studied red-billed seagulls, had told her the warm water meant krill in the ocean was pushed much further down, beyond the reach of gulls who normally fed it to their young.

Instead, the gulls were feeding their chicks fish larvae and jellyfish - "anything they can find", Ms Ballance said.