Today's the calm before the storm, or at least before the rain, for most regions around New Zealand.

The forecast for centres around the North Island today is for overcast skies and a chance of showers.

Auckland and Hamilton were expected to hit a high of 20C and Tauranga, Taupo and Wellington were a little chillier at 18C.

The South Island was cooling down a notch today after a scorching hot start to the week.

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MetService meteorologist Sarah Haddon said most regions were experiencing warmer weather because of a miniature heatwave caused by a high sitting to the east of the country.

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

The heatwave also caused thunderstorms across the South Island yesterday as well as sea fog in coastal areas like Dunedin.

Christchurch and Alexandra were forecast to hit 24C, down from temperatures around 26C earlier in the week.

Dunedin was expected to hit 21C, as were Invercargill and Queenstown.

The calm weather wasn't expected to stick around, and most regions can expect a lashing of rain heading into the weekend.

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

The low was expected to create a stream of warm, moist air from the subtropical oceans across the country.

This would likely produce showery rain for the West Coast of the South Island on Thursday night and would set in across the rest of the country.

A band of rain would linger over the North Island on Saturday and Sunday and stick around for the start of next week.