Sweltering temperatures which have broken all-time records in parts of the North Island will persist until at least Friday, forecasters say.

It would have been another restless night for many with few places across the country dropping below 20C, with high humidity.

Auckland was at 20C at 6am, Wellington Airport 21C, while Lower Hutt was the top of the bunch at 22.4C.

MetService meteorologist Mark Bowe said the toasty overnight lows were being driven by the warm, stagnant mass of air over the country.

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The hottest places today would be Hamilton, at around 30C, and Christchurch, on 31C. Auckland, which hit 30C yesterday, was forecast to drop only slightly to 28C.

The heatwave, which has pushed many parts of the country above 30C, has forced some schools to postpone their starting dates until after Waitangi Day to avoid the unrelenting heat.

In Wellington, unusually high temperatures overheated the overhead power supply to the train network late yesterday afternoon and forced 8000 commuters to take other forms of transport.

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MetService meteorologist Georgina Griffiths said the high temperatures would not drop until Friday in the South Island and Saturday in the North Island, when a southerly change arrived.

She said a "slab of hot atmosphere" originating in Australia had parked itself over the country, meaning high temperatures nationwide. The only places which were below 30C were coastal regions which had been cooled by marine air.

"Wednesday is only day three of this heat," Griffiths said.

"We've got five days of heat. It's important for people to realise the extent of this. You might get one day or two days like this in the past historically in New Zealand, but a run of five days is pretty notable."

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"I certainly think there'll be a few more records in the next couple of days."

Niwa meteorologist Ben Noll said parts of inland South Island including Hanmer Springs in Canterbury and Cromwell in Central Otago could reach 35C or higher by the end of the week.

Noll said the highest temperatures were being recorded inland because they were too far from the coast to be cooled by onshore sea breezes.

For that reason, Napier fell from 35C on Monday to 26C on Tuesday as the easterly from the Pacific Ocean swept in and cooled the region.

The hottest spot in the country yesterday was Kawerau in the Bay of Plenty, which peaked at 34.3C at about 4pm in the afternoon.

Kawerau I-Site manager Lizzie Te Rire said she had been advising tourists to head for the local swimming holes and rivers to cope with the heat. It was the warmest day she could recall in the town in her lifetime.

"I don't remember it being more than 31C here, ever."

In Wellington, a 90-year-old temperature record was broken. For about 10 minutes, the suburb of Kelburn rose to 30.3C - the highest since records began there in 1927.

On the roads around Taupo, road surface temperatures hit 60C - hot enough to fry an egg, said MetService.

Several principals said they had opted not to start classes until after Waitangi Day because of the heat at this time of year, combined with parents wanting to work up till Christmas at the other end of the year.

Freemans Bay School principal Sandra Jenkins, whose classes start on February 7, said it was too hot for children to sit in classes.

"This time of year is very, very hot. Imagine having kids in the classes this week" she said.

"It will be hot again next week, but at least it's not as long in terms of having those hot days."

Today's weather

Whangārei

Chance early drizzle, then fine until cloud develops in the evening. Southeast breezes. 26C high, 19C overnight.

Auckland

Fine and warm, some cloud morning and night. Easterly breezes. 28C high, 18C overnight.

Hamilton

Sunny and hot, but some morning cloud. Light winds. 30C high, 16C overnight.

Tauranga

Sunny, but some morning and evening cloud. Northerly breezes. 26C high, 19C overnight.

New Plymouth Fine apart from morning cloud. Light winds, afternoon westerlies. 25C high, 17C overnight.

Napier Fine and warm, but some early cloud. Northeasterly breezes. 27C high, 17C overnight.

Whanganui Morning cloud, then sunny and hot. Light winds, afternoon westerlies. 28C high, 18C overnight.

Wellington Fine and hot, apart from some morning and evening cloud. Northerly breezes. 25C high, 18C overnight.

Nelson Fine and sunny. Northerlies. 26C high, 18C overnight.

Christchurch Fine apart from morning cloud. Northeasterlies. 30C high, 16C overnight.

Dunedin Cloudy with chance light morning rain, then becoming fine. Light winds. 25C high, 17C overnight.