Most centres in the South Island are expected to top 30 degrees Celsius on Tuesday, as the spell of intense heat across the country enters its last few days.

Niwa Weather forecasts have suggested some inland places, such as Oxford and Waiau, could get very close to 40C, if they don't actually top that mark.

Very early on Tuesday, MetService was forecasting 37C highs for Alexandra and Clyde, but during the morning it dropped the expected high for the two towns to 36C.

123RF It's going to be a scorcher in the South Island.

"It's probably not a whole degree change," MetService meteorologist Tom Adams said. The drop in published forecast highs was more likely due to rounding after a change in decimal places.

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"It's still going to be the hottest day of the year."

Alexandra, has had daily highs in the 30s for the past seven days and is officially in a heatwave, as are other Central Otago centres Wanaka and Queenstown.

Wanaka is forecast to get to 33C on Tuesday, Queenstown to 31C, Christchurch 32C, Blenheim 35C, Timaru 31C, Dunedin 31C, Invercargill 30C, Gore 31C, Mt Cook 30C, Waipara 35C, Darfield 33C, Methven 32C, Culverden 34C, Omarama 32C, Twizel 35C, and Reefton 31C.

PETER GROVES Tuesday could well be the day when the highest temperatures of this summer are recorded. Things will be very different by Thursday for areas of Central Otago now in a heatwave.

"There is a good chance that stations in the Central Otago and Canterbury areas might record temperatures even higher than this, so be prepared for sweltering conditions," MetService meteorologist Kyle Lee said.

All the South Island, apart from Fiordland where it could be showery, is forecast to be mostly fine.

Most places in the North Island are expected to reach the high 20s, although a top of 34C is forecast for Masterton and 30C for Taumarunui and Dannevirke. Auckland is expected to get to 27C, Hamilton 28C, Tauranga a modest 25C, Napier 28C, Palmerston North 29c, New Plymouth 27C and Wellington 26C.

Most of the North Island is also expected to be fine, although isolated showers are possible just about anywhere, and thunderstorms are possible in the central high country after lunch.

MetService's Adams said he was sure some places around the country would reach 40C, but they were likely to be places such as backyards where the air was not moving around freely.

"In terms of genuine free air temperature, which meets MetService's criteria, I would expect places higher than we're recording, but whether they hit 40C or not is questionable."

Temperature depended on a range of factors. "To get really record temperatures, you have to have all these effects lined up in the perfect combination," Adams said.

"Today's the day they all line up the best."

Factors included plentiful sunshine, which played a big part in high temperatures, as well as the warm air - with contributions from all over the tropics and Australia - which had been sitting over the country for sometime.

"Places we expect to be the hottest are generally east of mountain ranges - in Canterbury, Otago and Masterton," Adams said. That's because of the foehn effect, when air comes over the mountains and warms as it drops down the eastern side.

"The foehn wind is a component of all the really hot days we get," Adams. And the more moisture in the air, the greater the foehn heating effect.

"The foehn heating effect is a consequence of moisture in the air condensing as it goes over the mountains. When water vapour condenses into liquid water and falls out that releases heat."

But if the foehn was so strong that the air became turbulent, mixing up the different layers, then it would lower the temperature, Adams said.

The main reason Tuesday was expected to be warmer than Monday was because the northwest foehn wind had picked up marginally. "Tomorrow the winds pick up more and become too strong."

On Tuesday morning, Niwa Weather was still considering the possibility 40C could be reached, after tweeting on Monday that one of its forecast models was showing an expected Tuesday high of 39.5C in the inland Canterbury town of Oxford.

"Nothing's really changed in terms of our thinking. We still think 40 is possible, if not close." Niwa forecasting principal scientist Chris Brandolino said.

On Monday, Cromwell was the warmest place in the country with a high of 36.6C, Niwa Weather reported. Lauder reached 35.8C and Clyde got to 35.5C. Last Thursday, Waiau in North Canterbury got to 37C - the highest temperature recorded in this country in seven years.

After a long run of hot days with plenty of sunshine, some stormy weather is moving in, with stark changes on the way for the South Island in the coming days.

The ridge of high pressure that has been keeping much of the country warm and sunny is expected to move off on Thursday, when, MetService forecasts, "the weather will turn dramatically".

That's because a front from the south and the remains of Tropical Cyclone Fehi. tracking down from the Coral Sea, are expected to come together then and cross the South Island, bringing heavy rain gale force winds across most of southern and central areas.

"The arrival of this severe weather will coincide with king tides around the country, which could also cause storm surges on the west coast of the South Island," Lee said.

"As the low crosses the South Island during Thursday, we're expecting gale force winds and heavy rain for most of the South Island, including drought-stricken Southland."

Heavy rain and gale force winds are also expected in western and central areas of the North Island.

In places such as Queenstown, the temperature is expected to be much lower on the last two days of the week, with a high of just 15C on Friday - half the temperature expected on Tuesday.

Invercargill is expected to be down to a 15C high by Friday, while Christchurch is expected to reach just 17C on Saturday, with Wellington looking at 19C on the same day.

In Auckland the decline in temperature by the end of the week will be much less noticeable, with a high of 24C expected on Saturday.