The hottest days of summer for the country are coming up, says Niwa.

"It wouldn't be a stretch to say what's going to happen late this week into early next week, in terms of the spatial size and the intensity, will be the worst of the summer heat ... certainly of what we've seen so far, perhaps for the season as a whole," Niwa principal forecasting scientist Chris Brandolino told RNZ.

"We'll see some really warm to hot temperatures today across the country."

After that, a "puff of cooler air" would "kind of tickle" the South Island, and temperatures may fall a bit, but then on Sunday and into early next week, some hot air coming off Australia would move over the country.

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"Air coming from Australia, hot to begin with, gets even warmer as it crosses the [Southern] Alps," Brandolino said.

BRADEN FASTIER Kahu Hunter-Brown of Nelson cooling off in Lake Rotoiti at St Arnaud, as hot, humid weather continues across most of the country.

Brandolino said it appeared the warmest temperatures would be on Tuesday in interior parts of Otago and Canterbury, south of Christchurch.

Temperatures in the upper 30s were a "distinct possibility".

"I wouldn't call it a slam dunk. When it gets to these extreme temperatures, the difference ... could be an hour of cloud cover, or a wind shift," Brandolino said.

To reach the limits of the temperatures the atmosphere could produce, things needed to go right, and even an hour or so of mid to high level cloud in the middle of the day could throw things off.

He was cagey about the possibility of the temperature getting to 40C though. "It's tough to give you a useful answer, right now... Right now, I'd say it's possible, not probable."

MEAN TEMPS 'INCREDIBLE'

MARTIN DE RUYTER If the office air-conditioning isn't up to scratch, Tahunanui Beach has been a fine option for Nelsonians looking to beat the heat.

Despite all the heat, so far January hasn't been particularly notable for record high temperatures. Invercargill got to a record 32.3C and Dunedin Airport got to a record 35C, although the Dunedin suburb of Musselburgh did reach 35.7C in January 1981.

According to Brandolino, the spectacular statistics so far this January have been more about mean temperatures. To get those, you add the high and low for the day, divide by two, and then average over all the days of the month.

"It's absolutely incredible," he said. " There are a plethora of locations on track for either the record, or near record (top four), warmest January."

Among those places are Kaitaia, Auckland, Hamilton, Tauranga, New Plymouth, Gisborne, Masterton, Hastings, Palmerston North, Wellington, Greymouth, Nelson, Christchurch, Oamaru, Wanaka and Invercargill.

Most seemed to be 2-3C above normal, with some 4C over, Brandolino said. To put that in context, the mean January temperature for Kaitaia is 19.5C, for Mangere in Auckland it's 19.6C, for Kelburn in Wellington it's 16.9C, and at Christchurch Airport it's 17.2C.

In some parts of the country, at least, warm nights were the probable reason for the high monthly mean temperatures, particularly places where humidity was high.

"When you have higher humidity, you tend not to cool off as much at night," Brandolino said.

So, for example, Auckland was on track for its warmest mean minimum January temperature, which at this was 3C above average, while the mean temperature overall for January so far was 2.4C above average.

In contrast, in Gore the mean maximum temperature so far was 6C above normal, while the overall mean was 4C above average. Gore recorded its fourth-warmest January day on the 14th, when the temperature hit 33.4C, Brandolino said. "That may go by the wayside next week."

RECORDS BROKEN

This summer so far has been remarkably warm, with the January heat following on from a scorching December when, Niwa records show, many places had their warmest mean temperatures on record for the month.

Those places included Nelson where records started in 1862, Levin where measurements date back to 1895, and Ranfurly, which has data from 1897.

Eleven places had their highest December temperatures, including Hanmer Forest with 34.6C, while Waiau had its equal-highest with 34.7C.

The highest temperature recorded in New Zealand is 42.4C at Rangiora on February 7, 1973. The North Island had its highest temperature on the same day - 39.2C at Ruatoria.

Meanwhile, Wanaka is in the midst of a heatwave - defined as five days when the maximum temperature is 5 degrees Celsius or more above the average maximum for the month - MetService believed. The Central Otago town was forecast to top 30C for the next six days.

HEAT, THUNDERSTORMS

In the North Island the heat and humidity would continue.

"The North Island will be very humid, and although not as warm, may feel more uncomfortable. Auckland is not forecast to drop below 20C overnight from Saturday for at least the following seven days," MetService said.

In Tauranga the temperature wasn't expected to drop below 20C for eight days. It would hit 29C on Thursday, MetService said.

A mostly fine Thursday was expected in Auckland with a high of 27C, and the temperature was expected to rise to 28C from Saturday through to Monday.

Wellington was headed for 26C on Thursday and should also be mostly fine. While a rainy day would bring cooler temperatures on Friday, the mercury would rise again to hit a projected 27C on Monday.

Christchurch was forecast to reach 30C on Thursday - after 32C on Wednesday - with a chance of a few showers from evening.

Masterton and Alexandra were forecast to get to 33C on Thursday, Gisborne 32C and Hastings and Blenheim 30C.

Added to that there was a thunderstorm risk for some places into the afternoon and evening on Thursday.

MetService expected eastern and central parts of the North Island from Bay of Plenty to Hawke's Bay to have isolated showers, mostly in the ranges, where some could be thundery.

Isolated showers were expected in Southland from late morning, spreading into Canterbury during the afternoon and evening. Some of those showers could be heavy and thundery.

TROPICAL CYCLONE ACTIVITY

Forecasters are watching a slow moving depression that is expected to deepen in the Coral Sea, with a high chance of developing into a tropical cyclone by Sunday.

MetService meteorologist Lisa Murray said the potential development was being monitored closely, but it was too early to be able to say whether it would affect New Zealand.

One of the three main models used by MetService was showing a possibility of wind and rain by the end of next week, but the other two didn't have that scenario.

"It may completely miss New Zealand, or may have an impact but not be severe, or it could have a big impact. Really that's the whole range, it could be anything," Murray said.

"Right now we have a ridge of high pressure over the country. If that stuck around and a tropical cyclone developed, and if the ridge was strong enough, it would protect us from it coming anywhere near us.

"If you get a trough over the country, it could potentially move down."