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.
Latest updates: Wintry weather blast
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.
Photos: First snow hits South Island
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:
North Island:
Desert Rd (SH1) from Rangipo to Waiuru
Rimutaka Hill Rd (SH2)
South Island:
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
