A special weather advisory has been slapped on the whole country and electricity companies have warned of possible power cuts.
Emergency refuge centres are being set up, motorists warned off the roads, and shoppers urged to stock up, but to refrain from panic buying.
Dunedin residents appeared to have taken heed of the warning, with at least one central city supermarket having run out of staples such as bread and some tinned foods by 7.30pm.
The MetService has snowfall warnings on alpine passes and meteorologist Dan Corbett believes snow will be "a major issue'' in the south.
It is expected to fall to sea level, "or very near it", from Southland to South Canterbury from late tonight into Thursday and Friday, he said.
In Southland and eastern Otago, snow is expected above 200m this afternoon, lowering to near sea level overnight. In the 15 hours to 9am tomorrow 15-20cm is expected to accumulate above 300m, and 5-10cm is expected down to 200m.
The snow level is expected at around 200m from about Christchurch northwards, and dumps of up to a metre are expected above 300m for inland parts of Canterbury and South Marlborough.
Christchurch Civil Defence staff are on stand-by, having met emergency services and key agencies yesterday to prepare for the expected bad weather.
The Civil Defence Emergency Operations Centre is being set up this afternoon and can be activated overnight if heavy snow falls.
Bitterly cold southeasterlies will bring damaging gusts up to 150km/h for the South Island's West Coast, and 130km/h for exposed parts of Marlborough and Nelson.
"These winds could disrupt travel, and severe wind chill will stress livestock,'' Mr Corbett said.
The weather has already disrupted flights, with all air traffic in and out of Invercargill cancelled late this morning. Flights into and out of Queenstown have also been affected.
A severe weather warning for strong to gale southerlies and very cold temperatures over the South Island are forecast to spread into the North Island on Friday and Saturday.
The North Island should see snow falling to about 600m from late Thursday into early Saturday, as south to southwest winds become very strong.
However, WeatherWatch head analyst Philip Duncan said flurries of snow could be seen at lower altitudes in Auckland, where there is a risk of hail and isolated thunder tomorrow.
"While the snow level will likely be 500m-plus, which is higher than the peaks in Auckland, downpours may be big enough to blow snow flurries down onto the tops of the Waitakeres, Hunuas and possibly Bombays,'' he said.
"However, the forecast for Auckland is very messy _ so we will need to update again in about 24 hours time.''
AA has warned drivers to stay off the flooded and icy roads if they can.
"If you have to go out, drivers should check the conditions of the roads on the AARoadwatch.co.nz website before setting off,'' said spokesman Mike Noon.
State Highway closures and warning
SH 87 Kyeburn to Outram: Closed due to snow.
SH 6,97 Kingston to Lumsden, Five Rivers To Mossburn: Closed due to snow.
SH 8 Twizel to Fairlie (Burkes Pass): Closed due to snow.
SH 80 Glentanner to Mt Cook Road: Closed due to snow.
SH 8 Lindis Pass: Chains essential - Closed to heavy & towing vehicles.
SH 8 Raes Junction to Clarksville: Chains Must Be Carried.
SH 94 Mossburn to Te Anau. No towing from Centre Hill Road to Gorge Hill ( Mavora Turnoff).
SH 85 Kyeburn to Omakau: Chains must be carried.
