Invercargill is set to swelter in 25C today, which would be its hottest October temperature on record.

The mini heatwave settles over the lower South Island today, where Central Otago's Alexandra is forecast to hit 26C today and tomorrow.

MetService duty forecaster Nick Zachar said Christchurch, Wanaka and Blenheim would also be basking in summer-like temperatures today, expecting 24C.

But it's Invercargill looking to break the record. On Sunday, it clocked up a new record high of 24C, but if it reaches 25C today - as forecast - it will set a new record for October.

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The average mean October temperature for Invercargill, according records between 1995 and 2002, is 14.6C.

"They've already beaten their highest max but if we get to 25C today, and it should just be fine all day, then it will beat itself again," Zachar said.

MetService meteorologist Sarah Haddon said the blast of heat was caused by a high sitting to the east of the country.

Haddon said it was unusual for the high to stick around for several days at a time and cause such a widespread increase in temperatures.

"A heat wave is when we expect temperatures 5 or more degrees above the usual, for five days in a row," she said.

The rest of the country, however, won't be as lucky. Temperatures in the North Island will be lucky to reach 20C.

"By and large the country is going to see some great weather," Zachar said. "But 25 is very warm for Invercargill."

It will mean showers for parts of the country, too: Auckland, Bay of Plenty, Coromandel, Nelson and Kaikoura are all expected to get brief rain today and tomorrow.

The warm temperatures and northwesterlies will also continue to bring sea fog to the coastline between Timaru and Dunedin, however, it's expected to push offshore today.

In Wellington, although the capital will only reach 19C, it will get a bit warmer inland where Hutt Valley is expected to reach 22C today and 21C tomorrow.

Meanwhile, Whangarei is waking up to a warm 16C. However, it's not expected to get a lot hotter and a high of 20C is expected today.

People should make the most of the warm and relatively dry weather before a front moves across from the Tasman on Friday bringing rain for the weekend.

According to Weather Watch, a low in the Tasman Sea will slowly approach New Zealand towards the end of the week bringing rain and a drop in temperatures with it.

Heavy rain will mostly affect the west coast of the South Island.

Showers will turn to rain along the entire western coastline of New Zealand on Friday.