New Zealand weather: More hot weather expected in Auckland, upper and central North Island

Those in the north hoping for cool relief from the hot and muggy weather are set for disappointment, with little change in the forecast this weekend.

Sun lovers will be smiling though, as warm air from the tropics keeps the mercury high.

Auckland reached the high 20s today and a repeat was expected tomorrow, MetService forecaster Sonja Farmer said.

A high of 27C is forecast - 5C above average - with partly cloudy skies and isolated showers in the afternoon and evening.

Sunday is expected to be mostly cloudy with a high of 25C.

Expect another busy day at Auckland beaches - these swimmers at Pt Chevalier Beach today were enjoying temperatures in the high 20s, which are expected again tomorrow. Photo / Alex Burton

It was a similar story in other northern and central parts of the North Island, where highs were reaching 2C or 3C above average for December, thanks to a northerly flow bringing warm air down from the tropics, Farmer said.

Hamilton can expect 27C tomorrow but drizzle on Sunday will peg the temperature at 24C.

Tauranga is forecast to reach 25C tomorrow and 23C - with drizzle - on Sunday.

Napier is heading for a scorcher tomorrow, with 29C forecast, dropping to 22C on Sunday.

Overnight temperatures are also soaring above average.

Napier was expected to drop only as low as 19C on Monday night, 6C above the average overnight low for the city in December, Farmer said.

Aucklanders who have already sweated through night-time temperatures as high as 20C this week can expect the pattern to continue.

"It's going to be 19C most nights this week."

Warmer than average daily highs are also expected to continue, despite wet weather on the way for much of the working week.

The lower North Island and the South Island are less affected by the tropical air mass, and showers are forecast over most of the South Island tomorrow, the heaviest on the West Coast.

A heavy rain warning is in place until 3pm tomorrow for Westland north of Bruce Bay.

Heavy rain watches are in place for Westland south of Bruce Bay, the ranges of Buller and the ranges of Tasman, west of Motueka over a similar period.

In contrast to many northern centres, temperatures aren't expected to exceed the teens in Christchurch or Dunedin this weekend, with Sunday forecast to be a particularly brisk 15C in both.

Nelson fares a little better, with 20C expected tomorrow and 19C on Sunday.

South Islanders could also expect to feel the first effects of wet weather moving across the country from the Tasman Sea from Sunday, Farmer said.

The complex trough would bring rain and showers to most parts of New Zealand, she said.