Amid assorted expletives, "remarkable" might be one word your average heat-beaten Kiwi would use to sum up January's relentless summer swelter.

It was precisely how Dr E. Kidson, then director of the former New Zealand Meteorological Service, described the summer of 1934-35.

Why is that relevant today?

Because, until now, climatologists have used the spring of 1934 and summer of 1934-35 as the record yardstick to measure our hottest seasons against.

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That record is now set to be sizzled.

Comparisons to date show the period from November through January has been running about 0.3C warmer than the 1934-35 event - and with the added influence of climate change.

Back in a time of depression-era American gangsters, and when Adolf Hitler was realising his dark vision of Nazi Germany, New Zealand was baking in drought conditions that lasted from the beginning of November to the middle of February.

"More remarkable, however, were the extraordinary high temperatures experienced," Kidson wrote.

November was described as a "summer month" being remarkably warm, with an average temperature of 14.9C - 1.2C above normal - and dry.

December 1934 was described as "remarkable", being 18.4C, 2.8C above average and even drier than November.

But January 1935 was the hottest ever experienced over the country, with a scorching 18.7C, which is 1.7C above average.

The drought was eventually broken by decent rainfalls that came in February, although temperatures - 18.5C or 1.3C above average - were still well above normal.

Nine-year-old Dredyn Abraham beats the heat with a jump into Northland's Waipu River. Photo / John Stone
Nine-year-old Dredyn Abraham beats the heat with a jump into Northland's Waipu River. Photo / John Stone

That summer weather reflected an active Australian monsoon season, filled with anticyclones that drifted over the South Island, generally creating easterly and northeasterly winds over the country.

Climate scientist Dr Jim Salinger said this latest November to January period had been warmer still, with an average temperature of 17.7C.

Although none us really needed to be told, January has been the standout month, with a mean temperature of 19.8C.

"It is already the hottest month on record, being 2.8C above average, exceeding February 1998 by 0.2C at this stage," said Salinger, the pioneer of Niwa's benchmark seven-station temperature series, and now an honorary research fellow at the University of Otago.

Still to come was a heatwave over the next few days tracking at 20.2C - a massive 3.2C above average.

What were in the ingredients in the atmospheric stew that made both these summers so hot?

Brian Phizack, 73, with his grandson Dylan McGregor, 11, at Okahu Bay in Auckland. Photo / Doug Sherring
Brian Phizack, 73, with his grandson Dylan McGregor, 11, at Okahu Bay in Auckland. Photo / Doug Sherring

Salinger said the one common factor was what's called the Southern Annular Mode, or SAM.

"The SAM is the most important source of variability in the atmospheric circulation over the South Island, and to the south in the Southern Ocean," he explained.

"This modulates the strength of the westerly winds in the roaring forties and furious fifties, and the subpolar oceans."

The strength of the westerly winds of the Southern Ocean was enhanced in the positive phase of the SAM and weakened during the negative phase.

"Thus, in a very positive phase as occurred in these two seasons the westerlies and their storms have contracted towards the Antarctic continent, allowing the big anticyclones to track over the South Island, and to the east."

The effect had effectively halted the mixing of cold Southern Ocean waters into the Tasman Sea and to the east of New Zealand, causing a marine heatwave that has pushed sea surface temperatures in the Tasman several degrees above normal.

But what made this event even warmer had been the background influence of climate change, Salinger said.

Since 1909, our annual average temperature has climbed by 1C - a figure in line with global average temperature increases, which are almost certainly the result of high levels of atmospheric greenhouse gases emitted from human activities.

2017 was the fifth warmest year since records began in 1909 - 2016 was the warmest, and others in the top five have all occurred in the last 20 years - and things would only get hotter as the planet warmed.

Under current projections, New Zealand temperatures could increase by several degrees by 2100, with climate change also bringing higher sea levels, higher ocean acidity, more floods, more frequent extreme storms, and more droughts.

New Zealand, which reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014, has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.