The cold snap is blasting up the country, with reports of snow falling in Nelson and Wellington tonight, forecasters say.
Snow was expected for most of the South Island and to low levels around the eastern and central North Island until tomorrow night, the MetService said today.
This evening WeatherWatch.co.nz had received reports of snow falling in Nelson, Motueka and Mapua, while it grew increasingly likely some light snow flurries would make it to sea level in Wellington, head analyst Phillip Duncan said.
''The snow reports coming in today are extremely rare and many people are commenting about how long it's been since snow fell to such low levels.''
There had also been reports of snow falling in Wellington's Karori and Kelburn suburbs, Mr Duncan said.
Hawke's Bay, Gisborne, Waikato, Waitomo, Taumarunui, Coromandel, Bay of Plenty and Taupo should expect snow to 300m overnight.
Residents in the lower half of the North Island also need to rug up, with snow falling to 200m, the MetService said.
The freezing conditions caused havoc on roads in the South Island today, with many closed or passable only with chains.
Invercargill police reported attending eight car crashes between 12pm and 2pm, caused by drivers travelling too fast for the conditions.
Senior Sergeant Dave Raynes, of Invercargill police, said many cars had left the road because of severe ice on State Highway 6, between Josephville Hill south of Lumsden through to Kingston, 48km south of Queenstown.
The bitterly cold blast has brought warnings for travellers and for farmers to get stock to shelter.
Queenstown Airport was closed this afternoon, with all flights in and out either cancelled, diverted or delayed.
The New Zealand Transport Agency said Dunedin's northern motorway, State Highway 1, was closed about midday.
Towing restrictions were in place and chains were essential for Porters Pass, SH93 from Clinton to Mataura, SH6 from Kingston to Dipton and SH94 between Milford and Mossburn.
SH8 from Lawrence to Milton was closed and there was heavy ice on the Pigroot - part of State Highway 85 from Kyeburn Road to Palmerston.
Sergeant Ian Kerrisk, of Alexandra police, said chains were essential for all vehicles and people should stay at home unless it was vital.
The first surge of cold air has moved over much of the South Island already but the coldest polar air was yet to reach Christchurch and with it heavier snow to lower levels, Mr Duncan said.
''This isn't the snow storm of the century...but what makes this system different from previous cold blasts is the low snow level, which is sea level across much of southern and eastern South Island and to very low levels right across the lower North Island. This is a national event,'' he said.
Snow flurries expected around Auckland tomorrow, and snow could fall on the Hunua Ranges, south of Auckland and on the peaks of the Waitakere Ranges. Any snow which fell on the Waitakeres was expected to quickly melt.
''Our focus is mostly on the rareness of snow falling to such low levels rather than predicting heavy snow for the main centres,'' Mr Duncan said.
Snow accumulations are likely to affect many roads and passes, these include most of those passing through the central North Island, the Napier to Taupo Road, the Gisborne to Opotiki Road, the Rimutaka Hill Road, the Lewis Pass, Arthur's Pass, Porters Pass, the Lindis Pass and Milford Road.