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.