You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
Drought-hit areas will likely have to go another week before there's any chance of relief.
Niwa forecaster Ben Noll says eastern parts of the South Island, particularly, could see temperatures soaring past 35degC on Saturday.
"I wouldn't be surprised if some places get to feel the effects of 37C. And that's preceded by another scorcher for Southland, Otago and interior parts of the South Island on Friday."
Noll said the warm trend would likely stretch on for seven days, into the back half of next week.
"And the hottest days of summer are still coming, from Friday, over the weekend, and into next week," he said.'
Latest maps showed how pockets of the North Island under meteorological drought, stretching from eastern Auckland to Northland, were expanding daily under the continuing dry spell.
For parched spots in the northern North Island, there wasn't any sign of rain for the next week at least.
"A lot of these places are going to easily get past 25C - with some odd spots getting up to 30C," Noll said.
The effect of heat in those driest areas would be compounded by the fact there was little moisture in the ground for the sun to evaporate - meaning its radiative force would be felt much more strongly.
"These places will have a week of this to contend with, and each day will be pretty darn warm," he said.
"We are keeping an eye on a low that may approach the North Island later next week, and put an end to the hot spell and bring rain, but it's still a little early to go into that."
While the Ministry for Primary Industries hadn't yet classified any region as experiencing an "adverse event", it was monitoring the situation closely, a spokesperson said.
"MPI is communicating regularly with the local Rural Support Trusts and local advisers, who are in touch with the sector representatives and farmers in the affected areas."
The forecast came as many councils had been forced to bring in water restrictions in many regions.
Water restrictions in South
OTAGO AND SOUTHLAND: Temporary conserve water notice for Alexandra. Sprinklers in Clutha district only allowed from 8pm to 8am. No restrictions in place in Gore, Dunedin, Queenstown Lakes district, Southland and Invercargill.
WEST COAST: Residents on Westport/Carters Beach water supply asked to restrict the amount of water they use each day. No restrictions in Grey or Westland districts.