Wanaka reached -3 deg C overnight, while Christchurch dipped to 2 deg C but it "felt like" -2 deg C, MetService said. By 6am today most of the lower South Island was shivering in near-below zero temperatures.
Highways in Otago and Canterbury were closed due to snow or icy conditions. In the North Island, the Desert Road (SH1) was closed from Waiouru to Rangipo.
The garden city can then expect a high of 12 deg C and low of 3 deg C on Friday. But it will climb to 15 deg C on Saturday and then 19 deg C on Sunday.
Waka Kotahi NZTA South Island journey manager Tresca Forrester said several roads were closed overnight to prevent people getting stuck in the icy conditions. They should reopen throughout today.
The Akaroa Highway (SH75) was closed this morning but reopened between Little River and Barrys Bay about 10.15am. However, winter driving conditions were still in place today.
Meanwhile, heavy snow warnings were in place for Southland, and much of Otago, the Southern Lakes District and Fiordland until mid-morning today. in the north, snow above 200m is forecast for Wairarapa and the eastern hills of Wellington.
NIWA meteorologist Chris Brandolino said more snow is likely to fall in eastern parts of Otago today.
"So Balclutha, Dunedin, they'll be dealing with snow showers, cold temperatures."
The core of the coldest air is likely to move over the South Island later today, he said.
"And then as that core of the coldest air shifts farther east, things will very slowly - I do emphasise slowly - improve today but a pretty windy cold day for the entire country."
The sub-zero temperatures would not be out of place in July, but are unusual in early October, he said. It was the first October snowfall to hit Christchurch in 53 years.
But the cold conditions are unlikely to last, he said.
"By early next week, say Monday, it's very possible that places that are dealing with snow, well everyone's dealing with the cold temperatures - places that dealt with snow this morning, for example Christchurch, they could be in the lower 20s."
In Christchurch, snow fell to sea level overnight and many cars and roofs were coated this morning.
The snow did not settle on roads, though, and there were no major issues getting around the city.
At Goughs Bay on the eastern side of Banks Peninsula, cattle farmer Sandie Stewart said her farm got about an inch of snow overnight.
The snow fell right down to sea level on the peninsula, Stewart said.
Sheep farmers who were still lambing might have some worries, she said.
- Additional reporting RNZ
