Severe thunderstorms could strike western areas of the North Island, including Auckland, tonight bringing heavy rain, hail, winds over 110km/h and even small tornadoes.

A front in the Tasman Sea is expected to move across central New Zealand overnight with a band of rain and embedded thunderstorms.

The MetService has issued a severe thunderstorm watch for Taranaki, Horowhenua, Kāpiti, Wellington and the Marlborough Sounds between about 10.30pm tonight and 2am Wednesday.

Auckland, Waikato, Waitomo and Taumarunui are under a severe thunderstorm watch from 2am to 6am Wednesday.

The purple pixels on the Westland radar show a line of thunderstorms south of Hokitika at the moment. https://t.co/prKU7O2R01 ^JL pic.twitter.com/OXnR90O97N

— MetService (@MetService) May 21, 2018

The thunderstorms would be accompanied by a short period of heavy rain, small hail, strong northwest wind gusts, and possibly a small tornado, MetService severe weather forecaster John Crouch said.

"Wind gusts of this strength can cause some structural damage, including to trees and power lines, and may make driving hazardous."

Meanwhile, an active cold front is moving northeast over the South Island and lower North Island today bringing heavy rain and strong winds.

⚠🌧💨 Severe Weather Warnings and Watches updated - now including the eastern Bay of Plenty. find the full details at https://t.co/qHyE5zhh6X ^JL pic.twitter.com/3GtqLDatoh

— MetService (@MetService) May 21, 2018

The heaviest rain is expected in Fiordland, Westland south of Otira, and the headwaters of the Otago and Canterbury Lakes and Rivers, with 100mm-150mm expected in places.

Northwesterly winds are expected to reach severe gale in exposed parts of the Canterbury High Country, Marlborough, Wellington and southern Wairarapa, with gusts up to 130km/h.

Active cold front producing rain & thunderstorms for the South Island.

Meanwhile, warm (relatively) unstable flow of air ahead of front is teaming up with separate weather system to produce showers/thunderstorms over the Tasman which will impact upper NI today.

~Chris pic.twitter.com/15sgUuxwF9

— NIWA Weather (@NiwaWeather) May 21, 2018

Snow is expected to fall to about 400 metres at times over the south of the South Island.

Many alpine roads in the South Island are subject to road snow warnings through to tonight including Lewis Pass (SH7), Arthur's Pass (SH73), Porters Pass (SH73), Lindis Pass (SH8), Crown Range Road, Milford Road (SH94) and Dunedin to Waitati Highway (SH1).

It's a wintry looking picture on the top of the Crown Range Road. Check the latest images here https://t.co/PtgQOeNclt ^JL pic.twitter.com/YoZ0w7DN15

— MetService (@MetService) May 21, 2018

Over the South Island tomorrow there is a moderate risk of thunderstorms about Kahurangi National Park, Buller and northern Westland through the whole morning, and about Fiordland and Southland before dawn.

These thunderstorms will bring brief heavy rain, hail, and snow to 700-800m in the north of the South Island and to 400-500m about Fiordland and Southland.

Tomorrow afternoon a very unstable west to southwest flow will cover northern and central New Zealand, bringing more heavy showers, thunderstorms and hail.

These thunderstorms are expected to be more frequent about the North Island west coast and Auckland region.