The wind chill will be below zero for exposed parts of Wellington, Christchurch and Dunedin with wind chills in some exposed parts of the South Island dropping to minus 15.
Farmers should be moving stock urgently to protect them from bitterly cold easterlies to southerlies, WeatherWatch said.
By Friday the storm will be pushing into the North Island with damaging winds possible along eastern areas. Snow may be heavy through Central Plateau and eastern areas may be cut off to the north and south if snow closes the major highways.
Travellers could face lengthy delays on Thursday and Friday as a result of the severe gales.
The warning has prompted Canterbury Civil Defence to urge residents to check their emergency supplies are fully stocked.
"Everyone should have the necessities in place to be able to be self-sufficient for three days or more in the event of road closures and power cuts," Regional Civil Defence group controller Neville Reilly said.
"This includes having stocks of food, water, warm clothing, fuel, alternative lighting and other emergency supplies on hand.
"We would also encourage people to make sure their homes are properly insulated. Installing heat tape on exterior pipes can help prevent water systems from freezing."
Christchurch City Council's transport and greenspace unit manager John Mackie said staff and contractors were out across the city helping to tidy up streets and clear drains following heavy rain.
"We are checking drains are clear and we have contractors briefed and on stand-by to respond to the snow if necessary. We are monitoring weather forecasts closely and will be ready if the snow arrives as predicted."
Christchurch Mayor Bob Parker said it was important for people living in areas at risk from snowstorms to consider the need for extra food supplies and alternative forms of heating and power generation.
The New Zealand Fire Service was also offering safety advise before the storm hit, including to keep everything at least one metre away from a heater or fireplace; avoid overloading power sockets or multi-boxes; and if using candles during a power cut, make sure they were in a stable, non-combustible candle holder and away from anything flammable.
The freezing conditions would bring an increase of risks on the road, police warned.
Specific Emergency Preparedness Items:
* torch with spare batteries or a self-charging torch;
* radio with spare batteries;
* first aid kit and essential medicines;
* blankets or sleeping bags;
* pet supplies;
* wind and waterproof clothing and strong outdoor shoes;
* food and water for at least three days;
* non-perishable food (canned or dried food);
* food, formula and drinks for babies and small children;
* water for drinking. At least 3 litres per person, per day;
* a camp-cooker or gas barbeque to cook on;
* a spare, full gas cylinder;
* a can opener; and
* water for washing and cooking.
Source: Civil Defence
- Additional reporting Kate Shuttleworth
