Hastings has recorded a new national temperature record, with a reading of 25.7C on Monday.
The unseasonable warmth also qualified as a new North Island temperature record for all of winter, based on Niwa records stretching back to the 1850s.
Niwa meteorologist Ben Noll said there were several factors behind the record-breaking high.
“We’ve seen an unseasonably warm air mass over New Zealand that originated in the Coral Sea. This combined with something called a foehn northwesterly airflow, which came over the North Island and warmed as it descended the terrain of the Central Plateau and blew into Hawke’s Bay,” he said.
“We also can’t ignore climate change - setting a new national temperature record for June is consistent with Aotearoa’s long-term warming trend, where we’re seeing fewer cold extremes and more warm extremes,” Noll said.
Temperatures on Monday reached above 20C for several regions, including Gisborne, Northland and other parts of Hawke’s Bay.
Conversely, maximum temperatures in the South Island struggled to get above 10C in places such as Christchurch and Queenstown.
Previously, the highest June temperature ever recorded was 25.4C on June 19, 1988, also in Hastings.
The reading also followed what was New Zealand’s coldest May in 15 years - something Noll said might have convinced Kiwis they were in for an unusually chilly winter.
“That’s a myth that can be dispelled, because those southerly winds that have been common over autumn are going to give way to more westerlies and northwesterlies, with air masses coming more from Australia than the Southern Ocean,” Noll earlier told the Herald.
Niwa is picking near or above-average temperatures for winter, beginning with a warmer spell, before a return to more typical conditions over the second half of June.
Noll said while there’d be cold snaps over the season, they’d likely be brief.
That change was coming with the demise of El Nino - the climate driver that’s been shaping our weather since spring, and which has been helping to bring up bitterly cold southwesterly flows from below us.
El Nino’s sibling La Nina - which delivered the North Island constant warm and wet weather for the first three years of the decade - is likely set to return, with Niwa giving a 60 to 70 per cent chance of the pattern forming up over spring.
In the shorter term, MetService was forecasting unsettled weather around New Zealand for the end, after a mild start.
“The end of the week looks set to see the return of some windier and wetter weather across many parts of Aotearoa New Zealand,” MetService forecaster John Law said.
“There may be the risk of severe weather, so remember to keep an eye on the forecast on MetService.com.”