It might be time for the first swim since last summer for some as temperatures approach a scorching 30C this week.

MetService meteorologist Tom Adams said a "good recipe" for hot temperatures was developing.

"We have a fairly moist air mass coming from the northwest, and northwest winds giving a bit of a foehn effect across the ranges."

The foehn effect occurred where an air mass released its moisture and produced heat as it passed over an area, leaving behind warm and dry air.

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"So we have got a lot of rain falling in the mountains and are left with this very dry and warm wind."

The hottest temperatures would hit the South Island's east coast on Wednesday and the North Island's east coast Thursday.

Today was forecast to be mostly fine across the country, with a warm northerly flow and temperatures around 20C up and down both islands.

The hottest spots for the week were looking to be Hastings and Napier on Thursday, with forecasted highs of 29C and 28C respectively.

In the South Island Christchurch would crack 26C on Wednesday and Dunedin 24C.

Western parts of the country would not get as hot as they escaped the foehn effect. Auckland would hover around 20-22C for the week.

Coming behind the northwest flow would be a cooler southerly change.

Christchurch would drop from 26C Wednesday to 12C on Friday.

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Southerlies would make their way up the country late Thursday into Friday, with some heavy rain in western areas.

By Saturday the southerly flow would have covered most of the country bringing cooler temperatures and showers to many areas.

By Sunday it was likely to settle for another nice day for most places.

Today's weather

Whangārei

Fine apart from evening cloud. Light winds. 22C high, 10C overnight.

Auckland

​ Fine for most, morning shower west coast. Southwest breezes. 20C high, 11C overnight.

Hamilton

Mainly fine, some morning and evening cloud. Light winds. 21C high, 7C overnight.

Tauranga

Fine. Northerly breezes in the afternoon. 22C high, 10C overnight.

New Plymouth Fine apart from morning and evening cloud. Westerly breeze. 18C high, 12C overnight.

Napier Fine. Light winds and afternoon sea breezes. 21C high, 10C overnight.

Whanganui​ Fine apart from morning and evening cloud. Northwest breeze. 20C high, 11C overnight.

Wellington Fine, then cloudy periods increasing. Northerlies strengthening. 17C high, 13C overnight.

Nelson Fine. Light winds and afternoon sea breezes. 19C high, 10C overnight.

Christchurch Fine. Northeasterlies developing in the afternoon. 21C high, 9C overnight.

Dunedin A fine day, then some evening high cloud. Northerlies. 19C high, 12C overnight.