Heavy snow is set to blanket southern and eastern regions of both islands over the next 48 hours and towering seas will surge through Cook Strait.

Extensive warnings have gone out as much of the country is set to freeze in the grip of giant snowstorm.

Shortly after 1pm snow began falling, though not yet settling, in the Queenstown Lakes district, with the council warning chains must be fitted for anyone driving on the roads.

READ MORE: Giant snowstorm: North Island now in firing line

Civil Defence authorities are telling people to prepare for bone-chilling conditions as the wild weather heads north.

The Queenstown Lakes District Council said there were two crashes - one on State Highway 6A near Yewlett Cres involving a truck rolling, and another on State Highway 6 near Boyd Rd.

The QLDC said temperatures were well below freezing with ice forming on all roads, especially through shaded areas and over bridges. Freezing fog in many places was adding to hazards.

Snow was falling on the Crown Range and in the Wakatipu Basin this morning, the council said, with MetService forecasting more throughout the day.

Travel operators across the country are preparing for major disruptions as the storm strikes. Flights have been cancelled at Queenstown Airport.

Air New Zealand senior external communications consultant Kelly Kilgour said "icy conditions" affected a number of services this morning.

The Interislander today cancelled all sailings from tomorrow evening through to Friday because of predicted 7m high waves.

Widespread snow is expected to fall as low as 100m across many southern and eastern areas of the South Island from now until Thursday.

MetService says Southland, Fiordland, Otago, Canterbury and Marlborough should prepare for "significant" snow falling below 500m.

In Otago, many roads have been affected by ice, with freezing fog also reported.

Snow showers were expected to affect the summit of the Dunedin to Waitati highway at times until Thursday morning. From 3pm today until 6am on Thursday, 2cm to 5cm could accumulate from time to time, especially on Wednesday.

In Southland, there were numerous road area warnings of ice and black ice.

Potentially damaging gales are also expected to whip through North Canterbury, Marlborough and Nelson tomorrow and overnight until Thursday.

Tomorrow the storm is due to hit the lower North Island.

Central and eastern regions are expecting up to 20cm of snow down to 400m.

Snow warnings, which have been issued for all South Island alpine passes and the highway north of Dunedin, now include Wellington's Rimutaka Hill Rd, the Desert Rd and the Napier-Taupo highway.

The lower half of the island is also expected to be pounded by gales and heavy rain on Wednesday and Thursday.

This morning snow was falling steadily in Central Otago as the rain band ushering in the frigid weather tracked up the island's east coast.

In Central Otago, heavy snow was falling in Manuherikia and Maniototo early this afternoon, making roads very icy.

The NZTA website advises SH85 from Becks to Wedderburn is closed because of snow. There are cautions for SH85 from Kyeburn to Palmerston and from Kyeburn to Alexandra because of black ice.

Oturehua and Ida Valley Omakau Road is closed, as is St Bathans.

The Danseys Pass Rd is closed.

Roads in Alexandra, Cromwell, Roxburgh and Ranfurly are open but care is needed.

Roads in Naseby, Kyeburn and Wedderburn were restricted to 4WD use only, and chains were to be carried.

In South Otago, there were patches of black ice on sealed an unsealed roads in the Clutha District. Black Bridge Rd (Clydevale) was particularly treacherous this morning, the council advised.

Icy roads in the Clutha District were the cause for a crash near Balclutha this morning.

Acting Sergeant Wayne McClelland, of Balclutha, said a van was driving towards Clydevale in Clutha Valley Rd when it went off the road due to the icy conditions.

Nobody was injured in the incident.

In North Otago, Acting Sergeant Stefan Witehira, of Palmerston, said police had no reports of crashes within North Otago yet, but he urged drivers to take care and drive to conditions.

The NZTA had an active snow and ice warning up for State Highway 8 between Omarama and Tarras.

An active ice caution notice was also up for State Highway 82 between Kurow and Waimate.Highways throughout the South Island were icy and care was needed, NZTA said.

Roads affected by ice included: SH98 Lorneville to Dacre in Southland; SH8 Clarksville to Raes Junction, SH90 Raes Junctions to Mcnab, SH1 Palmerston to Balclutha, SH87 Kyeburn to Mosgiel, SH85 Palmerston to Kyeburn and SH6 Arrow Junction to Kingston in Otago; State Highway 82 Kurow to Waimate and SH80 Aoraki Mount Cook National Park to Ben Ohau in Canterbury.

NZTA advised that alpine passes may all be affected by snow from midday on Tuesday, mainly on the higher parts of the road - Arthur's Pass and Porter's Pass (SH73), Lewis Pass (SH7), Lindis Pass (SH8), Haast Pass (SH6), the Milford Road (SH94). More snow may fall at Porter's Pass (SH73) from Tuesday evening through to Thursday - 15-20 cm or more near the summit.

"Please think carefully on whether you need to get in your car today and if you do, drive to the conditions - slowly, with chains for the alpine passes," NZTA journey manager Lee Wright said.

The Milford Road reopened about 11.30am.

Ahead of the stormy forecast travel operators began suspending trans-alpine services including the West Coast shuttle bus service, which has suspended its daily return journey tomorrow.

The Interislander said sailings would resume Friday morning.

'Worst storm' set to hit Otago

The "worst storm of the year'' was forecast for Otago today. Niwa climate scientist Nava Fedaeff said snow was expected to fall across the province for three days from today.

Most would fall on Tuesday, with up to 25cm in some parts, and the most exposed region would be East Otago, she said.

Snow was expected to fall in Dunedin and would settle on the hills, but snow falling at sea level "would not stick'', Miss Fedaeff said.

Driving conditions would be hazardous today.

"Be careful on the roads. It has been cold and icy the last few days and now there is snow in the mix.''

The storm would be the worst of the year, but she doubted it would break records.

"July is the heart of winter when you get your big storms and freezing temperatures. So far, it has been cold, icy and fog has caused disruptions [in Otago] but it's not in the record books.''

When and where the worst weather will strike

Snow falling below 500m

Southland and Fiordland: During today through to Wednesday afternoon.

Otago: Until around midday today, and again from tonight until Wednesday evening.

Canterbury and Marlborough: From this evening until Thursday afternoon.

Snow below 1000m
Taihape and Hawke's Bay ranges: From Wednesday afternoon until Thursday morning.

Severe gales:
Northern Canterbury, Marlborough, Nelson, Wellington and Horowhenua Kapiti Coast: From Wednesday afternoon until overnight Thursday.

Heavy rain
Wellington and Coastal Marlborough: During Thursday.
Wairarapa: From Thursday morning until Friday morning.