A brutal polar blast hitting the country today is set to bring sub-zero temperatures feeling as low as -15C, putting people and animals' lives at risk.

Forecaster Niwa has this morning issued a stark warning saying the incoming wintry blast is set to plunge the South Island at freezing levels with wind chill making it feel anywhere between -5 to -15C.

It says this will threaten a "real hypothermia risk for people and livestock".

MetService says a sudden "Antarctic blast" is set to chill the country over the next few days, starting with a blast of cold polar air.

In Canterbury the mercury is set ot drop a staggering 15-20C within a matter of hours today.

Heavy snow watch

A period of heavy snow is expected for the south and east of the South Island as what is tipped to be an intense cold outbreak spreads northwards over the country from this evening through to Thursday.

It will bring unseasonably low snow to the South Island and the lower North Island, the weather authority says.

"Snow is likely to affect many parts of the South Island and some roads over the lower and central North Island."

A heavy snow watch will be in place in Marlborough south of the Clarence River, the Canterbury High Country and foothills, also North Otago, Central Otago from Alexandra northwards and the Lakes District from Queenstown northwards.

The watch is in place from 10pm (tonight) until about 8am tomorrow.

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People in those areas are told that a brief burst of heavy snow is a possibility and snowfall amounts may approach warning criteria above 400m.

There is also a heavy snow watch in place in the Canterbury coast and Plains - including Christchurch and Banks Peninsula.

That watch starts from 1am tomorrow until 3pm on Thursday. Again, a brief burst of heavy snow is forecast from the early hours of tomorrow morning between 1am and 8am.

Locals are also being told that snow amounts may approach warning criteria above 400m.

Southland, Stewart Island, Clutha, Dunedin, Central Otago south of Alexandra, the Lakes District south of Queenstown and Fiordland from Te Ānau have are affected by the heavy snow fall watch too.

People in those parts of the country are advised the watch is in place for 38 hours from about 7pm until 9am on Thursday.

'Bitterly cold' temperatures this week

MetService meteorologist John Law said temperatures will drop across the country.

"We are all going to feel this very cold air when it arrives mid-week.

"While temperatures in the central parts of the South Island will be the coldest, even up in the winterless north we could find temperatures cold enough for a touch of ground frost."

He said day-time temperatures will struggle to break double digits for many parts of the country on Thursday, in particular, with strong south-westerly winds whipping around coastal parts of New Zealand.

"The windchill is going to make it feel colder still."

In Auckland, the coldest nights this week will be Thursday and Friday - with both nights forecast to have overnight temperature lows of 6C.

Wednesday night is not exactly warm either, however, with a temperature low of 8C. Saturday and Sunday have overnight lows of 9C in the City of Sails.

Today's forecast

Aucklanders are waking up to a crisp Spring morning, with the temperature measuring just over 12C shortly before 7am.

MetService recommends three layers of clothing today - although a high of 18C is forecast today and an overnight temperature low of 11C.

A partly cloudy day is expected for Northland, Auckland and the Coromandel. There may also be isolated showers.

For those in Waikato to the Kapiti Coast - including the central high country and Hawke's Bay - conditions are set to be fine or will become fine this morning. Cloud and isolated showers south of Taranaki is the forecast later this evening.

In the Bay of Plenty and Gisborne, cloudy conditions with a few showers will clear to become a fine afternoon.

People in Wellington, Wairarapa, Nelson and Marlborough are in for a fine day apart from areas of morning cloud and frosts. Buller is in for a few showers.

Westland and Fiordland can expect cloud increasing. Showers will spread north this morning and will turn to rain in Westland, with heavy falls possible.

Road snowfall warning

Motorists are being advised to keep up-to-date with road snowfall warnings that may affect their travel plans or journey.

MetService is due to issue a road snowfall warning for the Napier-Taupō Rd, Desert Rd, Remutaka Hill Rd, Lewis Pass, Arthur's Pass, Porters Pass, Haast Pass, Lindis Pass, Crown Range Rd, Milford Rd or Dunedin to Waitati Highway whenever there is a likelihood of snow setting on one or more of those roads within the next 24 hours.