Niwa has labelled the weather event as a bomb cyclone, or weather bomb, which is when low pressure strengthens at a pace of 24+ hectopascals (hPa) in 24 hours.
MetService is forecasting even more snow to fall across much of the deep south until later this afternoon after regions were coated in some of the heaviest snow seen in years.
The police and New Zealand Transport Agency are warning motorists in southern regions to avoid all non-essential travel as the wintry blast coats roads in snow, making driving conditions treacherous.
"Please check our traffic maps before you head out and be well prepared for cold conditions and slippery roads with poor visibility," said NZTA Southland and Otago system manager Graeme Hall.
He said crews were keeping an eye on key places, but people still needed to drive to the conditions.
"If you can delay your trips until the sun is out and ice is melting, that will help. On bridges and shaded areas there will be black ice, so take it slowly. Any areas that are gritted, slow down," he said.
The Southern District Police Facebook page added that "if you don't need to travel please stay home. If you can delay travel ... or not travel at all today, that is even better."
Invercargill is expected to dip to a frigid -10C, according to Niwa. Snow is down to sea level, coating beaches along the Dunedin coastline.
Today the North Island is also in store for a spell of polar weather, with snow expected to fall on the Central Plateau in the coming hours. A road-snow warning was issued for the Desert Rd earlier but has since been lifted.
A MetService forecaster said there was still a likelihood of snow showers but it would no longer be settling on the road.
The weather over the island's volcanic region would continue to improve across the day, with the chance of snow becoming increasingly remote into the afternoon.
Potentially damaging gales of up to 120 km/h are set to howl through eastern regions, with Hawke's Bay and Tararua District under a strong wind warning.
Strong winds are also barrelling through Auckland today, causing the Auckland Harbour Bridge to close during morning rush hour, though there are no weather alerts in place.
MetService forecaster Dan Corrigan said the city was in for a blustery day, with winds remaining strong and not expected to ease up until later on.
While snow hadn't fallen yet on the summit of the Desert Rd, snow showers were expected this morning.
"By far today is the more major day, before it starts getting better," said Corrigan.
He said it was unusual for Buller to see snow down to such low levels, though it wouldn't necessarily be down to the coast.
In the North Island, gale-force winds with the potential to cause significant damage and be hazardous to motorists would batter Hawke's Bay south of Hastings and the Tararua district until 9pm.
Weatherwatch.co.nz said gales might be damaging in Auckland, Otago and Southland for a time but southerners would have the additional impact of blizzard conditions and sub zero wind chill.
It follows a day of travel chaos across the South Island that saw airports close and motorists stranded.
With the southern regions turning into a winter wonderland, farmers were keeping a watchful eye on lambing ewes, while orchardists prepared to ward off frosts this morning in a bid to save their budding fruit crops.
