Half a dozen main highways across the country have been closed because of snow and ice.
Snow was falling in parts of the South Island this morning to 100m above sea level, and was forecast later in the day to 500m in the central North Island.
Sleet has been reported in Dunedin and snow flurries in Christchurch.
The Desert Rd (State Highway 1) from Rangipo to Waiouru had been closed overnight, and most of the South Island's alpine passes had been affected.
MetService meteorologist Nick Zachar said the freezing, Antarctic blast had made its way up the country overnight, arriving in the North Island mid-morning.
"Showers have been falling as snow to 200m in the lower South Island, but those should start to ease through the morning," Zachar said.
Cameras at Lindis Pass (SH8) and Crown Range Rd in the lower South Island showed snow completely covering the roads.
The NZ Transport Agency said snow has closed SH8 from Cromwell to Omarama in central Otago, SH94 Hollyford to Milford Sound, SH93 Clinton to Mataura in Otago, SH87 Outram to Middlemarch west of Dunedin, SH7 Hanmer turnoff to Springs Junction (Lewis Pass) in Canterbury and West Coast, and the Desert Rd from Rangipo to Waiouru.
Warnings were also in place for Dunedin to Waitati Highway (SH1), the Crown Range Rd, Haast Pass (SH6), Lindis Pass (SH8), Porters Pass (SH73), Arthurs Pass (SH73) and Rimutaka Hill Rd (SH2) on the North Island's east coast.
Zachar said as the cold front arrived in Wellington mid-morning it would bring showers and strong winds.
There was a risk of snow for the Rimutaka Hill Rd and more snow for the Desert Rd in the afternoon.
Snow showers were expected to 500m in the central North Island this afternoon.
Snow has been reported on the Napier-Taupo Rd (SH5).
Up to 10cm of snow could accumulate above 700m by 9pm, with lesser amounts to 500m.
Along with the cold southwesterly flow was a moderate risk of thunderstorms this morning for Buller, the far west of Nelson, Horowhenua/Kapiti Coast, Taranaki and in the west of Waitomo, Waikato and Auckland.
These would persist in northern areas through the afternoon and potentially spread to Northland.
The storms could be accompanied by strong wind gusts up to 110km/h, and hail.
There was also a moderate risk of thunderstorms in the Wairarapa after the southerly change this morning.
Temperatures plummeted this morning as the Antarctic flow moved up the country.
About 5.30am was Hanmer Springs, at -2.7C.
Mt Cook Airport dropped to -2.9C at 7am, Tekapo to -4.6C and Mt Hutt -10.1C.
Temperatures were forecast to drop again tonight.
"As the skies clear we will be in for a rather chilly night, with those southwest winds directing Antarctic air across the country," Zachar said.
Wednesday and Thursday were looking like much of the same, before a high pressure system began to form and slowly thaw the country out over Friday and Saturday.
Showers, some heavy with hail and chance thunderstorms from afternoon, easing at night. Brisk southwesterlies. 14C high, 5C overnight.
Showers, heavy with hail and chance thunder from late morning, clearing evening. Strong southwesterlies, gale gusting 100 km/h. 12C high, 4C overnight.
Showers, some heavy with hail and chance thunderstorms from late morning, clearing evening. Southwesterlies easing. 11C high, -2C overnight.
Fine spells. A few showers from late morning, chance heavy afternoon, clearing evening. Strong southwesterlies. 12C high, 2C overnight. Showers, some heavy with hail and chance thunderstorms from late morning, clearing evening. Southwesterlies easing. 11C high,
New Plymouth Showers, chance heavy with hail and thunderstorms, clearing evening. Brisk southwesterlies easing. 12C high, 1C overnight.
Napier Partly cloudy, chance shower from about midday with a strong southerly change. 12C high, 2C overnight.
Wellington Showers developing this morning with a strong, cold southerly change. Some showers could be heavy with hail. 8C high, 5C overnight.
Nelson A few showers clearing to fine this morning. Gusty southwesterlies. 12C high, 1C overnight.
Christchurch A few showers, with snow to 400 metres, clearing by evening. Strong southwesterly. 8C high, 3C overnight.
Dunedin A few showers, mainly in the morning with snow to low levels at first. Gusty southwesterlies. 8C high, 5C overnight.