Weatherwatch.co.nz is forecasting the snow to fall down to sea level across Southland and Otago tomorrow night with virtually nowhere in the South Island escaping a wintry white-out.
Travel across the country is expected to grind to a halt and farmers are being told to move livestock ahead of the wintry blast.
Coastal regions are being put on alert with possible flooding from strong onshore winds, high waves and heavy rain colliding in the storm. Sailings across Cook Strait could be in jeopardy with Niwa predicting waves of up to 4m by Thursday.
Snow is expected to fall for two straight days across the South Island - in some areas down to 100m in southernmost regions and down to 200m in Canterbury - followed by a bitterly cold southerly wind.
This morning a swathe of road snowfall warnings for major alpine passes and Dunedin's northern highway have been issued for coming days.
Projections show some alpine areas will get up to 1m of snow over the coming 72 hours.
On Wednesday the weather will continue to worsen as it tracks north with severe gales, heavy rain and snow set to strike the centre of the country.
Gales are expected to batter Taranaki to Gisborne south and heavy rain is expected to fall across the lower half of the North Island.
Snow is expected to bring disruption to central and eastern areas with forecasters predicting it will fall down to 600m across Wairarapa, Hawke's Bay and east of Taupo.
Weatherwatch.co.nz says the Wellington region could be hit by snow flurries down to 500m by dawn on Wednesday.
The weather is not expected to let up until Friday when it moves away from the North Island.
MetService forecaster Ciarin Doolin said with the wild weather is expected to affect many parts of both islands and people could expect roads across the country to become snowbound.
"This front is expected to bring significant severe weather, affecting many parts of New Zealand from Tuesday to Wednesday, including heavy rain, snow and severe gales, and may cause disruption to transport and stress to livestock. It is also worth noting that for coastal areas exposed to a strong onshore flow, high waves and heavy rain could lead to inundation," said Doolin.
"There is still a degree of uncertainty in the computer models regarding the mid-week severe weather event, so people are advised to keep up to date with the latest Severe Weather Outlook. It is highly likely that Severe Weather Watches or Warnings will be issued nearer to the event."
Canterbury's Civil Defence Emergency Management warned residents to prepare for the approaching storm.
In previous years, heavy snowfall and gales have led to road closures and power outages affecting large parts of Canterbury, the group said.
Motorists considering driving over mountain passes in winter were advised to take chains with them in case of snow and to check road conditions and the weather forecast before setting out.
Meanwhile, a perfect storm of rain and freezing temperatures in Otago and Southland made for some of the most treacherous driving conditions in decades, the Otago Daily Times reports.
Police responded to more than 40 crashes across the South at the weekend and warned drivers to stay off the roads on Saturday morning and last night. Police put out warnings about Southland, Central Otago and the Waitaki last night.
The coldest temperature recorded by the MetService was -6.3C in Alexandra on Saturday morning.
The MetService is forecasting freezing temperatures to continue. Frosts were expected this morning, and snow to 200m in Otago tomorrow.
Lawrence Volunteer Fire Brigade chief fire officer Rory Tisdall said driving conditions were at the "most extreme" on Saturday morning.
"I have been in the Lawrence brigade for 24 years and the conditions over this weekend were the most serious I've seen," Tisdall said.
On Saturday, temperatures of about -6C in places, combined with rain about 6am, created the "perfect storm" for severe black ice, he said.
The most serious of the crashes he attended involved a van which rolled after hitting ice on State Highway 8 near the Beaumont Hotel and Holiday Park about 10.30am on Saturday.
The van was being used by Green Island Football Club's premier team to get to a game in Queenstown and teenager Matt Milton was airlifted to hospital after suffering serious "crush injuries" to his hand, shoulder and leg in the crash. The game with Queenstown was called off.
Sergeant Brent Kingsland, of Invercargill, said the same conditions had caused "havoc" in Southland and it was "very lucky" there were no serious injuries.
"There were vehicle crashes all over the show," Kingsland said.
He had worked in Invercargill for 16 years and the icy conditions were the worst he had seen.
Police in Southland attended about 28 crashes on Saturday.
In Queenstown a man who was clocked at 179km/h in Gibbston on Saturday was charged with driving at a dangerous speed.
Acting Senior Sergeant Jared Kirk said the 40-year-old was caught speeding in a 100km/h zone on SH6, near Victoria Flats Rd, about 3.25pm.
The man had his licence suspended immediately, and will appear in the Queenstown District Court on July 24.
"There is still black ice and loose grit across many of our roads, so to hear of driving behaviour like this is extremely disappointing.
"It's clear the message is not getting through to some drivers."
Some things to consider, from Canterbury Civil Defence:
Review your travel plans for the week; avoid driving if possible, otherwise keep driving to a minimum.
Allow extra travel time if you do need to drive.
Do a thorough check of the vehicle and all equipment before driving. Travel with extra clothing, food, water, and a phone charger.
