A wet weekend is on the cards for some with a front making its way up the South Island tonight and smothering the North Island by Sunday.

In the North Island the rain will only hit the lower areas late Saturday, while the northernmost areas will be dry until Sunday.

MetService duty forecast Matthew Ford said today the North Island weather was relatively settled, with just a few showers in western areas.

A thick layer of fog had formed overnight from Waikato through to the Bay of Plenty after a rainband crossed the North Island, followed by clear skies.

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Temperatures were relatively mild overnight, for winter at least, with the coldest temperatures -5C at Pukaki in the Canterbury high country, -4C at Middlemarch in Central Otago and -3C at Dunedin Airport.

Where there were clear skies in the South Island there would be frosts this morning, Ford said.

It would prove to be a mostly fine day out east, while the West Coast started to feel the impacts of a moist northerly flow ahead of a front arriving tomorrow.

A severe rain warning was in place from 9pm tonight to 9am tomorrow about Fiordland north of Doubtful Sound, and Westland south of Franz Josef Glacier, with up to 160mm of rain forecast.

The heavy rain was a sign of things to come for the country, with a front spreading rain west over the whole of the South Island Saturday, before reaching the lower North Island in the evening.

The North Island was in for a mostly dry Saturday, with most places forecast for fine weather and temperatures in the mid teens.

On Sunday the front would arrive spreading rain across the entire island.

Behind the rain band it could get quite cold in the south with a southwest change, Ford said.

"On Sunday in the South Island the rain will be easing to snow, and they could get snow to 500m."

These southwesterlies and showers would spread over the whole South Island and later the North Island during Sunday.

The showers would persist in southern and eastern parts of the South Island on Monday, with lowering temperatures, and snow was expected about the hills and ranges.

There could be heavy snow above 300m about Southland, Otago and southern Fiordland.

On Tuesday, the west to southwest winds were expected to weaken over the country, with easing showers.

Cooler temperatures were expected over the North Island for early next week.

Today's weather

Whangārei

Long fine spells. Southwest dying out morning. 16C high, 5C overnight.

Auckland

​ Often fine. Isolated morning showers in the west. Southwest dying out morning. 15C high, 6C overnight.

Hamilton

Morning fog, then sunny spells. Light winds. 14C high, 3C overnight.

Tauranga

Some morning cloud, then fine. Light winds. 15C high, 5C overnight.

New Plymouth Cloudy periods. A few light showers, clearing from afternoon. Northwest breezes. 14C high, 8C overnight.

Napier A frosty start to a fine day. Light winds. 16C high, 2C overnight.

Wellington Cloudy periods. A few showers in the north. Northerlies picking up. 13C high, 10C overnight.

Nelson Crisp and clear, with morning frosts. Light winds. 14C high, 2C overnight.

Christchurch Fine with morning frosts. Northerlies. 12C high, 1C overnight.

Dunedin Fine. Increasing high cloud from afternoon. Northerlies. 14C high, 8C overnight.