Most of the country can expect a sweltering Sunday, with temperatures in the mid to high 20s, MetService said.
Taumarunui in the central North Island will peak at a high of 31C, and further north, Auckland and Kaitaia could both expect highs of up to 29C.
Castlepoint in the Wairarapa had already reached just below 26C before 10am on Sunday.
But the heat on Sunday could be mixed with scattered showers in many places, forecasters said.
A heavy rain warning is in place for Tairāwhiti north of Tolaga Bay for Sunday and into Monday.
Duty meteorologist Surprise Mhlongo said the area could see up to 175 millimetres of rain between 6pm Sunday and midday Monday.
Rainfall that led to a state of emergency in Westland on Friday and Saturday has eased overnight.
Rain or showers was expected for much of the country on Monday, MetService said.