Freezing weather moving across the country is threatening to wreck havoc on South Island roads, while thunderstorms look to hammer the top of the country.

State Highway 6 between Haast and Makarora was closed from 1.30pm due to snow falls and SH94 had two warnings in place since this morning - between Mossburn and Te Anau as a result of ice on the road and between Te Anau and Milford due to snow, the Automobile Association said.

A snow warning was also in place on Crown Range between Wanaka and Arrow Junction since midday.

The MetService warned of snowfall on a number of Canterbury and Otago roads.

On Lewis Pass and Arthurs Pass, rain was expected to turn to snow tonight before easing to showers tomorrow morning.

The MetService said two to three centimetres of snow could fall to 800 metres on Lewis Pass, and five to eight centimetres on Arthurs Pass down to 700 metres, in the six hours from midnight tonight.

On Porters Pass, snow was expected tonight with three to five centimetres expected down to 700 metres and light snow to 400 metres in the 12 hours from 2am tomorrow before easing tomorrow afternoon.

At Lindis Pass, up to eight centimetres of snow was expected to fall down to 400 metres in the 15 hours from midnight tonight, easing tomorrow afternoon.

On Milford Road, rain was expected to turn to snow tonight with five to eight centimetres of snow expected to 700 metres tonight and two to three centimetres down to 500 metres tomorrow.

Meanwhile, a severe thunderstorm warning had also been issued for Northland, Great Barrier Island, Coromandel and the Bay of Plenty.

Thunderstorms accompanied by strong winds were expected in the northwest of the North Island tonight and in the Bay of Plenty tomorrow morning.

WeatherWatch head weather analyst Philip Duncan said the thunderstorm would be fast moving, and had already hit over the Tasman Sea.

WeatherWatch's lightning tracker had clocked more than 2000 strikes in the Tasman Sea today, he said.