A severe low is set to hit southern New Zealand tonight, before moving north.

MetService said the low would bring 150km/h winds and thunderstorms to southern parts of Fiordland, Southland, inland Canterbury, Marlborough, Wellington and Wairarapa.

MetService spokesman Dan Corbett said the low was expected to pass over the South Island overnight.

"This low is moving really fast and deepening quickly as it approaches southern New Zealand.


"It's expected to bring a period of strong winds to parts of the country as it passes by," Mr Corbett said.

The northwesterly gales are then forecast to spread over Wellington and Wairarapa, with gusts reaching 120km/h, while easing over the South Island.

Southern parts of Fiordland, Southland and Stewart Island were most likely to be exposed to damaging winds.

"We're looking at gusts of 150 km/h in the period between midnight Monday and 6am Tuesday.

"Gusts of this strength can blow roofs off houses, bring down power lines, break large branches off trees and blow vehicles off roads," Mr Corbett said.

Expected to hit near Stewart Island, the low is expected to bring severe thunderstorms, "hurricane-force" winds and squalls and some of the biggest seas on the planet, according to WeatherWatch.co.nz.

Snow to low levels in the deep south is also predicted, along with possible tornadoes and waterspouts along the western coastline.

However, WeatherWatch said the centre of the low will just miss New Zealand.

"This is a nasty storm and if we plonked the current weather map over the tropics you'd swear you were looking at a tropical cyclone" said head analyst Philip Duncan.

"However, it's anything but tropical with cold air at its core and it will help dredge up some very cold air from well south of New Zealand".

Mr Duncan said the low met the category of "weather bomb", meaning air pressure would fall by 24 hectopascals in 24 hours.

Southern and eastern areas of the South Island would be hardest hit, with the worst-affected areas the rural and less populated regions, he said.

Emergency Management Southland manager Neil Cruickshank advised residents in coastal areas of Southland, including Invercargill, to ensure all loose roofing iron was secured, loose items of outdoor furniture put away or tied down, and items such as trampolines moved to sheltered locations.

Residents should also prepare for power cuts.

"We want people to stay at home and indoors from about 10 o'clock tonight until daylight tomorrow," Mr Cruickshank said.

"If the gales develop at the strength MetService is forecasting, it will be dangerous to be outside with flying debris in the air and trees or branches falling across roads. Be prepared to delay travel plans on Tuesday morning if the winds are still dangerous."

Southland's harbourmaster Kevin O'Sullivan urged boat owners to check moorings.

"There are 6m swells forecast in Foveaux Strait, and Bluff Harbour will certainly be affected," he said