Temperatures in Rotorua and the Bay of Plenty soared this week, with several centres reaching 30C or above.
According to Niwa, the hottest temperature in the country on Monday was 32.1C, which was recorded at Kawerau, followed by 32C at Whakatane.
Rotorua weather man Brian Holden said Rotorua has an unusually high temperature of 30.8C
He said although it was a high temperature for the city, it wasn't a record.
Mr Holden said Rotorua's highest maximum temperature on record was 35.2C which was "some time way back in the early 1900s''.
The city also managed to reach 31.5C in January 2004 and January 1983.
Niwa climate scientist Georgina Griffiths said there had been only 21 days warmer than Monday's maximum, since records began 100 years ago.
"It's a notably warm day,'' she said.
It was the hottest day this summer, far surpassing the 27.7C recorded on December 19.
"That's a big jump. The Bay of Plenty often gets 28C but anything over 30C is unusually warm ... it's not surprising people are running for the beach.''
But the scorching conditions will make way for heavy rain and gales this week, as as a west to southwest flow spreads across the country, said MetService.
MetService forecaster Philippa Murdoch said the maximum temperatures yesterday were likely to sit in the mid-20s.
"Basically the warm temperatures over much of the country over the last couple of days have been due to a very warm air mass spreading over the country from Australia. Now it's starting to move away and there is more of a south westerly flow over the country bringing in a bit of slightly cooler air,'' she said.
The front is expected to bring rain, stronger winds and gales to central parts of the country and exposed parts of Northland and Auckland before moving to the South Island.