Heat alerts are possible as temperatures rise across the country, with Ashburton expected to hit 31C.

MetService meteorologist John Law says Wednesday started off warm with a muggy night and continue to heat up in most places as the day progressed.

"Do keep an eye on yourself, make sure you stay hydrated, watch out for your friends and family and do seek shade in the heat of the day as well."

Some wet weather could be expected to head towards Fiordland through to Stewart Island, says John.

"Some heavy falls of rain are not out of the question in through Fiordland, but you can see most of the wet weather being kept way down across the southern parts of the country and for many of us, it's a dry and fine day and a hot one as well.

"That's really the most amount of rainfall we'll find on these charts over the next day or two. For many of us, it's a dry looking story beneath that high pressure still lingering across the top of North Island."

John says the eastern side of both the North Island and South Island would really catch the heat.

The east coast of the North Island could reach 29C, while Christchurch could hit 30C and Ashburton, 31C.

Auckland is expected to hit 25C and Hamilton, 27C.

Looking ahead to Christmas Day, MetService's extended forecast is looking dreary for many places.

Tauranga's forecast is for a cloudy day and a high of 25, with an overnight low of 18.

Auckland is expected to be partly cloudy with rain developing and northerlies while Kaitaia's forecast showed rain, turning to showers and northerlies.

Wellington is predicted to have rain with strong northwesterlies, and Christchurch was expected to have showers, clearing to some cloud and light winds.

Queenstown is expected to be fine with southwesterlies, and Dunedin is the same.

-RNZ.