Despite the polar blast bringing freezing temperatures for many Kiwis last week, last month was the hottest June on record.
Nationwide average temperatures were 2C above average, something that has only happened 13 times since 1909, according to the National Institute of Water and Atmospheric Research (NIWA).
The record-breaking warmth comes as temperatures in other parts of the world have also been unseasonably warm recently. A "heat dome" in Canada and the United States' Pacific north-west led to temperatures reaching almost 50C last week in some places normally known for having moderate climates.
According to NIWA, the warmth in June was widespread across New Zealand, "with every long-term monitoring station observing either above or well above average mean temperatures".
The highest temperature was 22C, which was recorded in Hastings on June 26 and in Leigh on June 19.
Motueka was also particularly warm, the figures showed, recording a mean temperature of 10.8 - 3.2C higher than the town's 1981-2010 average.
A total of twenty four places across the country had their warmest June on record, with NIWA attributing the increase in temperatures to more northeasterly air flows than usual.
"The prevalence of these air flows, occasional low pressure systems that transported warm, humid air down from the subtropics, and ongoing background warming from climate change meant it was a very warm start to winter throughout the country," NIWA said.
Across the six main centres, Auckland was the warmest and sunniest, Tauranga was the wettest, Christchurch was the coldest and driest, and Dunedin was the least sunny.
Rainfall was also above normal or well above normal for much of the country, though there was less rain for parts of Central Otago, south and mid Canterbury, Nelson, Tasman, inland Whanganui, Gisborne and eastern Bay of Plenty.
The warmer start to summer comes after New Zealand recorded its seventh warmest year on record in 2020.
While globally, last year was the second hottest year on record for the Earth, with average land and ocean surface temperatures 0.98C above average.
The continual rise in temperatures has led to increasing concern from scientists who say governments need to take action urgently to stop global warming.
"The risks have been understood and known for so long and we have not acted, now we have a very narrow timeline for us to manage the problem," former UK chief scientific adviser Sir David King told The Guardian earlier this month.
He said the extreme heat in Canada and the US recently showed that "nowhere is safe" from the effects of climate change.
"Who would have predicted a temperature of 48/49C in British Columbia?" he said.
On Tuesday (local time) the Canadian town of Lytton recorded the country's highest-ever temperature when the mercury hit 49.6C.
Maximum June temperatures in the town are usually around 25C.