Hawke's Bay had the top three warmest temperatures in the country on Monday, with recording-breaking highs in Napier.
However, residents sweltering under the near 30C temperatures will be relieved to hear a polar spell will bring cooler weather, and some rain, later in the week.
On Monday afternoon the city reached 27.8C while in Hastings reached 28.1C.
"That's half a degree hotter than the record from 2004," MetService meteorologist Alwyn Bakker said.
The previous maximum temperature recorded in Napier for the month of May was 27.3C, recorded in 2004.
NIWA said this was provisionally May's fourth warmest temperature for the entire country since records began in 1869.
Waipawa also broke records for May, reaching 26.9C.
Hastings' records were not available at the time, but Bakker said it seemed likely Monday's temperatures would also be record-breaking or close to it.
Bakker said the region was experiencing a little hotspot.
"It's still pretty warm in the rest of the Bay area but is particularly hot around Napier and Hastings.
"What's been happening is we've been getting a lot of warm, moist air from the Northwest, Tasman Sea.
"Because that's coming from the equator it's very warm."
The weather was a little cooler further south in Dannevirke, with temperatures only reaching 21.7C while Wairoa only reached 24.7C.
Bakker said the warm weather would continue until Tuesday night before starting to cool down from midweek onwards as a cold southerly rolled in.
"We've got 16C for Monday night and then 11C on Tuesday, going down into single digits overnight for the rest of the week.
"We're getting that really abrupt temperature change because we're swapping from quite a tropical air mass to quite a polar air mass."
The cold spell on Wednesday would also bring "a good dose of rain" to the region, welcome relief to many of the region's farmers.
While Monday's temperatures were record-breaking for May, rainfall across the region was well-below March and April averages at about 40 per cent.
Rain from north of Wairoa to south of Pōrangahau averaged just 40 per cent of April averages, and parts of southern Hawke's Bay had less than 20 per cent.
Rain maps for the past two months highlight the most widespread deficits of rain in Hawke's Bay are the most severe in the past five years, on top of a significant drought last year.