The Desert Road and Rimutaka Hill Road are closed by snow tonight, as the cold blast makes its way up the country.
Further south, roads and schools closed and mail deliveries were cancelled today due to icy roads.
Air New Zealand said thunder and lightning delayed flights at Auckland airport, snow had affected services to and from Dunedin, and heavy snow showers caused widespread cancellations at Queenstown.
Alexandra had seen one of the heaviest falls in years, while snowfall in Kingston, Otago, exceeded 30cm, WeatherWatch analyst Richard Green said.
Areas of Canterbury and Wellington received light snowfall and cold southerly winds today with hail and sleet.
Chains were required in most places in the south, and motorists were warned to take extra care particularly over the Lewis, Porters, Arthur's and Lindis Passes.
Some Dunedin schools were closed today and bus services cancelled due to snow, and mail deliveries were halted across most of Otago.
The conditions forced two outpatient clinics to close at Dunedin Hospital.
The Rimutaka Hill Road between Wellington and Wairarapa closed due to snow about 9am, with up to 20 vehicles stuck on the road. The road opened this afternoon but further snowfalls were expected and the road was closed again tonight.
Heavy snowfalls were forecast for Taihape and the central North Island high country, MetService said, with 25-35cm accumulating by morning.
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A tornado reportedly struck in Port Waikato, tearing off roofing iron, WeatherWatch said.
Some 300 homes were without power for a time in the Auckland suburbs of Takanini, Bethells Beach, Henderson and Muriwai after the city was hit by thundery showers and blustery, cold winds.
Swells of more than eight metres were recorded along Auckland's western coastline today, and weren't expected to ease until Thursday or Friday.
The wild weather also toppled a large tree onto a house in the Auckland suburb of Pakuranga, and cut power to 450 homes in the Far North.
MetService said the storm would ease tomorrow.
Roads closed as at 8pm today:
Desert Rd (SH1) from Rangipo to Waiuru
Rimutaka Hill Rd (SH2)
SH 87 from Middlemarch to Outram
Severe weather warnings:
*Heavy snow above 300m in Southland, Clutha, south and southeast of Fiordland, central Otago
*Heavy snow in Taihape, central North Island high country, ranges of Wellington and Wairarapa
*Gale southwesterlies in Auckland and Northland