Northland's warmest May on record has been followed by a record warm June in places and the experts predict the rest of winter is going to be unusually warm too.
In its climate summary for June, NIWA said the month was the third warmest on record across the country, with some towns, including Kaikohe, recording some of its warmest June temperatures ever.
NIWA principal scientist - forecasting Chris Brandolino said, during last month, air pressure was higher than normal over and to the east of New Zealand - leading to warmer, northerly winds.
"In addition to frequent winds from the north, warmer than usual sea surface temperatures persisted around New Zealand, especially to the west and north. Much like the past several months, these two factors worked in tandem to bring unusually mild temperatures to the country," Mr Brandolino said.
And NIWA is forecasting, in the three months from July to September, there is a 70 per cent chance of temperatures in Northland being above average.
Last month, Kaikohe recorded its warmest June mean air temperature since records began in 1973 at 14.1C, which is 2.3C above the June norm.
The town also recorded its second highest mean maximum air temperatures for June at 16.5C, or 1.1C above the monthly norm.
Kaitaia and Whangarei also recorded their second highest mean maximum air temperatures for June at 17.5C (1.3C above normal) and 16.4C (1.6C) respectively.
Kaikohe was seemingly the place to be weatherwise as the town also recorded its second highest mean minimum air temperature for June at 11C, or 2.2C above normal.
Kerikeri, meanwhile, recorded its fourth highest daily temperature for June since records began there in 1981 with 21.1C on June 10.
Kaikohe recorded its second highest June daily high of 16.6C on June 10; Kerikeri recorded its third highest daily temperature on the same day of 16.5C and Whangarei had its third highest June daily temperature of 16.3C that day also.
Kaitaia recorded its fourth highest daily June temperature the same day at 16.5C.
July started with the coldest day of the year so far overnight Friday/Saturday. The overnight low on Friday/Saturday at Kaitaia hit 0.3C.
Whangarei had an overnight low on Friday/Saturday of 2.3C and a low overnight Saturday/Sunday of 1.3C.
Kerikeri's low overnight Friday/Saturday was 1.7C, but it dropped lower overnight Saturday/Sunday to 1.3C.
The temperatures were likely to have been even lower elsewhere in Northland.
NIWA says between July-September temperatures are likely to be above average in all regions of the country, but frosts and cold snaps will occur from time to time.