Snow has started to fall across the South Island and gale-force winds have toppled trees and powerlines as a fierce polar blast takes hold.

A severe weather warning is in place for heavy snow in the far south of the country as the first significant cold snap arrives with a vengeance.

Up to 20cm of snow is expected to begin falling this afternoon. It will last for the next 18 hours in southern Otago and Southland hills.

Some alpine roads are expected to be impassable with snow falling down to 200m today.

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Alexandra will get the worst of the winter chill overnight with temperatures set to drop below -2C.

The MetService has updated its forecast predicting snow to fall near to sea level in Dunedin today.

Bitterly cold gales of 120km/h are also expected to lash the south.

The wintry weather is expected to hit the North Island tomorrow where the Desert Rd is now expecting 3cm of snow tomorrow morning.

Snow is falling in Central Otago between Omakau and Ranfurly and motorists are being warned to take extra care on highways.

There was a light dusting of snow in the township of Naseby overnight.

A light dusting of snow fell in Naseby overnight. Photo / Craig Sherson
A light dusting of snow fell in Naseby overnight. Photo / Craig Sherson

The MetService says heavy snow is expected to coat parts of the Southern Lakes and Central Otago from the Crown Range southwards, Dunedin, Southland and Clutha.

Fierce gales are battering eastern regions and monster 6m swells will pound the country's western coastline today. The MetService said the biggest waves could easily double to 12m in height - and be incredibly powerful.

Colossal 9m waves are due to hit Puysegur Pt in the far southwest of the South Island this afternoon.

People are being warned to be careful on coastal roads at high tides.

The Interislander ferry is warning passenger sailings across Cook Strait may be cancelled this weekend because of the predicted giant waves.

The Bluebridge Cook Strait Ferry has suspended passenger bookings from Saturday afternoon until early Monday.

Firefighters say they were busy overnight securing roofs and clearing trees and powerlines felled by the destructive gales from Canterbury to Wairarapa.

A Fire Service spokesman said it had about half a dozen callouts across the Canterbury region between 7.30pm and 1am as the winds picked up.

In the North Island trees and powerlines came down across roads east of Woodville.

But the worst weather is reserved for the next 24 hours, and snowfall warnings have been issued for all South Island alpine passes and the Dunedin to Waitati highway.

Snow is expected to coat Queenstown today as temperatures dive from 10C to below zero.

The NZ Transport Agency said the highway linking Te Anau to Milford Sound was likely to be closed overnight and up to 15cm of snow is expected today.

Queenstown Lakes District Council said the Crown Range had a light dusting of snow overnight.

The road was open but motorists were asked to carry chains or take an alternative highway.

Here are the current Warnings and Watches for wind and snow. Wrap up warm its going to be a cold couple of days. You...

Posted by MetService New Zealand on Wednesday, 17 May 2017

The MetService said the worst weather will arrive today as bitterly cold air direct from Antarctica sweeps up the country.