Heavy snow is already pelting parts of the South Island today, with blizzard conditions expected in some places.
Snow has been falling all morning at Mt Cook, and there are reports of heavy snow at Glenorchy near Queenstown.
MetService meteorologist Derek Holland said Southland and Clutha had heavy snow warnings in place.
"Arthur's Pass, Porter's Pass and Milford will all have issues with snow today," he said.
"For Otago today we're expecting falls of 20-30cm at about 400m."
Even though this winter has been warmer than normal, a polar blast cools things down across most of New Zealand for the next few days in what is expected to be the coldest month of the year.
The Canterbury Plains, Christchurch, Central Otago, the Southern Lakes, Dunedin, Clutha and Southland are subject to a severe weather watch issued by MetService, with an expected 20 to 30cm of snow expected to fall to 400m and a dusting below that.
Temperatures plummet in the North Island, too, and Aucklanders should break out the puffer jackets as the daytime high in the city of sails on Saturday will barely break double figures.
"There will be heavy rainfalls across the North Island," Holland said.
"With northwesterly winds it's not that cold, so we're not expecting any snow for the North Island."
Heavy rain is expected in the lower North Island and upper South Island, with Tarurua, Kapiti, Wellington and Wairarapa on the receiving end of the worst of it.
Yesterday's relatively mild temperatures will seem a distant memory by the week's end as the South Island freezes and overnight lows drop to -4C and -9C in the worst-affected inland areas on Friday, according to MetService meteorologist Arno Dyason.
Southern winter conditions shroud populated areas as well, as Timaru wakes up to -4C.
Umbrellas may be redundant moving into the weekend, but winter woollies are a must as the southern air flow moves north.
Commuters may find their travel disrupted as well over the next few days, as the New Zealand Transport Agency issues snow warnings for State Highway 73 over Arthurs and Porters Passes, and for SHs 8, 6, 94 and 1, between Dunedin and Waitahi.
Knowledgeable locals normally carry chains and roading authorities recommend this for snow-covered roads.
Southern officials are also warning motorists to be careful of snow and icy conditions elsewhere in the South Island, including the invisible black ice.
Central Plateau residents up north are also in for snow, with a severe weather outlook issued by MetService warning people to expect flakes to fall above 700m about Tongariro National Park and Taihape on Saturday.
Other parts of the North Island will remain chilly for a couple of weeks in what is expected to be a prolonged wintry patch.
Coming after a wet period, Auckland's drop in temperature may experience some relief over Saturday and Sunday, as heavy falls ease to chilly showers.
Knocking on the door of spring, skiers who might otherwise be thinking about the final few weeks of their season can instead expect a good run of snow, with Mt Ruapehu expected to be a popular destination this weekend.
Today: Showers turn to rain, 18C
Tomorrow: Rain at times, 16C
Today: Showers turn to rain, 17C
Tomorrow: Showers, 15C
Today: Rain develops, 15C
Tomorrow: Occasional showers, 14C
Today: Rain develops, 17C
Tomorrow: Showers, 15C
Today: Rain develops, 14C
Tomorrow: Rain at times, 11C