Those hot nights are set to become more tolerable as wet, cool weather sets in across the North Island.
A low-pressure system has been making its way up from the South Island, bringing with it rain which will set in this week.
"We have a cold front, a low, a southerly change and patchy rain and showers affecting both islands over the next few days," WeatherWatch said on Sunday.
While some areas are expected to have just a little drizzle or a few showers, other parts of the country will have heavier rain with a chance of thunder.
The risk of thunderstorms is considered moderate about Buller and western Nelson on Monday morning and about Taranaki later in the morning, MetService says.
There is also a "moderate risk" of thunderstorms during the afternoon and evening in Auckland, Coromandel, Bay of Plenty and Gisborne down to Manawatu and the Tararua District, as well as Marlborough in the south.
"The rain will come as a cool change to some of the high temperatures we have seen across the country over the last few days," MetService meteorologist Mark Bowe said.
"Napier got to 33degC on Saturday which is very close to the highest January temperature on record for that area", Mr Bowe explained, and those high temperatures are set to ease this week.
Auckland and Wellington have seen temperatures higher than normal January averages, according to MetService. Some South Island towns also saw temperatures reach higher than average at times during last week.
"The southwest change will cool temperatures down a little as the wet weather moves across the country," Mr Bowe said.
He said there is "plenty to keep an eye on as changeable weather is expected".