A climate scientist says our summer is on track to be New Zealand's hottest ever - and warmed ocean surfaces have helped "rev up" Cyclone Gita.

Aucklanders sweated last night as temperatures failed to fall below 22C - but the hot evening was just another among many that have left countless Kiwis sleepless this summer.

Climate scientist Dr Jim Salinger said the New Zealand temperature (NZT) series up to February 20 was tracking at 18.7C for the month - 1.5C above average.

That also put the summer's average temperature so far at 19C, or 2.4C above average, said Salinger, an honorary research fellow at the University of Otago.

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"The previous hottest summer was that of 1934/35, when NZT was 18.5C - So this summer is a full 0.5C warmer than the previous [record]," he said.

"If the forecast temperatures until the end of February are included, the February value comes in at 1.1C above average, and the summer value comes in at 18.9C, 2.2C above average, and 0.4C more than that of 1934/35."

An associated marine heatwave that set in over October had persisted, and sea waters that were the warmest at this time of year were "providing the fuel to rev up cyclone Gita", Salinger said.

"The warmest summer on record - which goes back to the 1860s - has produced marine impacts such as many bluebottle jellyfish, and decline in algal blooms, kelp forests with warm water fish being very prominent.

"Wine grapes are about three weeks ahead in their development compared with last year."

Salinger said the drivers had been a very positive Southern Annular Mode, which had kept the Southern Ocean storms at bay, with no cold southerly outbursts and injections of cold southern ocean water into the New Zealand region.

The background drivers of climate change would also have had an influence, he said.