A freak marine heatwave and record-hot temperatures that warmed up New Zealand last summer is a taste of our possible future under climate change, scientists say.

The unmistakably warm season brought hellishly humid nights, a top-heated Tasman Sea, the hottest single month on record, localised droughts, a boom in insect numbers and the damage and deluges of cyclones Fehi and Gita.

It also caused the biggest-ever melt on the Southern Alps, which lost nearly 10 per cent of their ice over the season.

A corresponding marine heatwave pushed sea surface temperatures to between 2.5C and 4C above average through much of December 2017.

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Incredibly, some localised spots off the West Coast even reached between 4C and 6C above normal.

A paper just published in scientific journal Environmental Research Letters found last summer - which broke an 84-year-old record for warmth - could also offer a good analogue for possible mean conditions in the late 21st century.

Its authors – including prominent scientists Professor Jim Salinger, Professor James Renwick, Dr Erik Behrens and Dr Brett Mullan – said the extreme summer could be typical of an average New Zealand summer climate for 2081-2100, under mid-range projections.

Salinger described the effects as "dramatic", with a record early snow melt, major disruption of marine ecosystems and very early maturation of wine grapes.

For Marlborough Sauvignon Blanc wine grapes, the 2018 season proved to be amongst the warmest since 1947, with véraison and maturation dates advanced by 13 and 17 days respectively.

Salinger put the warmth down to a combination of factors, including a persistently positive Southern Annular Mode which brought fewer cold fronts, a La Nina climate system, and background global warming.

The many southern anticyclones calmed the waters in the Tasman Sea and east of New Zealand, allowing heating of the upper layers of the ocean.

"It covered a huge area the size of the Indian sub-continent," Salinger said.

"So it was unprecedented in 150 years of temperature observations in land and sea in the New Zealand area."

He added it also flagged a warning for the future.

A separate study published yesterday found the water around the country was significantly warmer than it was three decades ago.

The strongest warming had occurred off the Wairarapa Coast, and the weakest along the northeast coast between North and East Capes.

Niwa scientists behind the paper, which analysed ocean temperature changes over the past 36 years, expect this trend only to continue and have pointed to climate change as an obvious driver.

"Since 1981 we are talking of warming of about 0.1C to 0.3C per decade," oceanographer and lead author Dr Phil Sutton said.

"That may not sound like a huge amount but slightly stronger warming of about 0.4C per decade off the east coast of Tasmania has resulted in significant changes to ecosystems which has led to concerns of similar impacts in New Zealand."

Sutton said an obvious cause of the long-term warming trend was climate change.

"There is nothing here that is out of step with what has been projected to happen," Sutton said.

"We would expect to see this continue – perhaps the warming in the northeast will continue to be weak but you would suspect the Wairarapa Coast to be vulnerable to further change."

"The 30-year time series is long enough to see long-term changes and there is no reason to think this will turn around."

Niwa scientists were now closely watching another event unfolding.

Over past weeks, average sea surface temperatures had climbed more than 0.7C above the long-term average, with waters being observed around the South Island and to the west, south, and east of the country.

Sea surface temperatures were closer to average near Northland and more broadly to the north of the country.

While they were generally warmer than average, they were still markedly cooler than this time last year, when the record marine heatwave event was underway.