Kiwis have been given a heads-up for a “surge of humidity” as the tropics fire up.

An incoming wave of humid air will cause “especially muggy” conditions towards the end of the week in the North Island, Niwa said.

“Both islands are in for a surge of humidity,” it warned.

“All that extra moisture isn’t just for show either, expect some rain in places, thanks to the tropics firing up just north of us.”

MetService meteorologist Rob Kerr told the Herald humidity was set to stick around in much of the North Island due to an absence of wind or northward cold fronts to push warm, humid air out.

“If we have fairly light winds like we have under a ridge, we’re kind of stuck with that muggy air for much of the week.”

Kerr said a cold front provided respite for parts of the South Island early in the week, although warm air and muggy temperatures would come into effect afterwards.

Meanwhile, rain is set to hit parts of the lower South Island tonight before moving further north, Kerr said.

“We’ll see some scattered rain affecting parts of the South Island through tomorrow morning and then clearing through the afternoon and evening.”

A cold front will keep temperatures relatively cool in the South Island while it moves northward, he said.

Nelson is forecast to reach 24C today, with Christchurch set for 17C and Dunedin 18C.

In the North Island, a westerly flow is forecast to bring light rain to central regions in the early morning before easing into isolated showers in the afternoon and early evening.

Scorching temperatures in the North Island are set to ease slightly today, with Napier and Hastings expected to reach highs of 28C and 29C respectively.

Whakatāne, under a heat alert for this weekend, is forecast to hit 29C before dropping to 18C overnight.

Elsewhere, Auckland is set to reach 25C, Hamilton has a predicted high of 26C, while Wellington is set for a cooler 19C.