Wrap up warm folks, as the heavy snow is only just getting started.

The South Island is in lockdown as most of the country hunkers down to ride out the polar blast.

Flights across the country have been cancelled, ferry services across Cook Strait have stopped and people are being advised to stay indoors as snow and wind lash both the North and South Islands - with the worst expected to hit tomorrow.

South Island motorists have been told to stay home as snow and ice leaves roads too dangerous to drive on. There is a heavy snow warning for inland mid Canterbury, north Canterbury, Kaikoura and the central North Island high country, including ranges in Hawke's Bay and Manawatu.

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The Desert Rd in the North Island and Lewis Pass and Arthur's Pass in the south are all closed overnight.

Gale force winds are expected in Wellington and Wairarapa, which could reach up to 140km/h or more. In addition to the wild winds, significant rainfalls are expected around the lower North Island, with 100-50mm in the hills and ranges and 70-100mm around lower lying areas.

Those planning to cross the Cook Strait are out of luck after InterIslander and Bluebridge cancelled all their sailings. They are expected to resume Friday morning.

The snow forecast for the North Island will worsen tomorrow and MetService has extended its heavy snow warnings to include the Manawatu ranges and Taumarunui as the "significant" winter storm bears down on the centre of the country.

Even the north of the country looks like it won't escape the approaching wild weather with gales threatening to lash Auckland and Northland tomorrow.

The New Zealand Transport Agency (NZTA) says South Island roads are in a treacherous state and the safest place for vehicles is in the garage. It's also worried about people travelling through the central North Island with heavy snow expected to fall on the Napier-Taupo Rd and Desert Rd.

"If you are able to, postpone your journey," said NZTA journey manager Lee Wright.

"In many places snow is lying on top of ice, making driving extremely dangerous."

MetService advises there's a heavy snow warning in place for inland mid and north Canterbury and the Kaikoura District.

Further snow is expected tomorrow and early Friday, and is expected to ease by late Thursday afternoon. Higher parts of Kaikoura above 500m may get up to 50cm of snow.

A heavy snow warning is also in place for the central North Island high country, including the Hawke's Bay and Manawatu ranges. Heavy snow is expected above 600m from this afternoon through to Thursday afternoon, with lesser amounts down to 400m.

Weatherwatch forecaster Philip Duncan said a deepening low to the east of Hawke's Bay was pushing moisture into the cold southerly air, creating heavy snow.

"Our forecast rain radar is showing this heavy snow event for the North Island is a Thursday/Friday event," Duncan said.

Airports across the country are experiencing delayed and cancelled flights, with passengers advised to check online to avoid being stranded.

All regional flights from Wellington International Airport were suspended today.

A police spokeswoman said they had been called out to "at least 10 crashes on State Highway 8 yesterday and today as weather has deteriorated".

"Police would like to remind road users to reduce their speed and have chains fitted where appropriate."

Temperatures are expected to be as low as -6C in Queenstown, and may even drop to -10C through the alpine interior.

The weather is expected to cause widespread disruption to transport, and could lead to flooding and slips.

Road snowfall warnings

These roads are expected to be hit hard by snow tomorrow:

Napier-Taupo Rd (SH5)

Five to 10cm of snow is expected to accumulate on higher parts of the road.

Desert Rd (SH1)

30 to 40cm is likely to accumulate at parts of the road.

Rimutaka Hill Rd (SH2)

15 to 25cm expected to accumulate.

Lewis Pass (SH7)

Snow is expected to affect the road until tomorrow afternoon, with 15 to 25cm of snow falling about the southern and eastern parts of the road.

Arthur's Pass (SH73)

Further snow showers tomorrow night could add a further 1 to 3cm on higher parts of the road.

Porters Pass (SH73)

20 to 30cm of snow is expected to fall until 6pm tomorrow evening.

Lindis Pass (SH8)

Light snow showers will leave 1 to 3cm of snow settling on the road.

Milford Rd (SH94)

A few snow showers are possible south of Lake Gunn, with 1 to 2cm of snow expected.

Dunedin to Waitati Highway (SH1)

Snow showers could affect the summit of the road tomorrow night, with 2 to 3cm accumulating.