Temperatures are forecast to plummet, with Christchurch reaching just 9C tomorrow and Queenstown 8C and -1C overnight Tuesday.
Snow is expected from Monday night into Tuesday about higher inland parts of the South Island with heavy falls likely. Queenstown and Wanaka could have snow down to lake-level, and places like Naseby were likely to get heavy snow.
Snow is also likely about the higher roads of the North Island later on Tuesday.
There is a heavy snow warning in place for the Canterbury high country to 2pm tomorrow and 6pm in the Kaikoura ranges.
Snow is forecast down to 400m above sea level in Canterbury, with pockets of snow as low as 300m. Over 40cm of snow may fall in areas 700m above sea level.
In the Kaikoura ranges snow is forecast to 400m, and over 40cm may accumulate in areas over 600m.
Road snowfall warnings have been issued for the alpine passes in the South Island, and also for the motorway north of Dunedin out to Waitati.
For Lewis Pass, Arthurs Pass and Porters Pass, snow is forecast above 400m, with the largest accumulations at higher levels.
For the Lindis Pass and Haast Pass as well as the Crown Range Rd and Milford Rd, snow is forecast down to 300m, with the largest accumulations at higher levels.
The Dunedin to Waitati Highway is also expected to see a dusting of snow, with a few cm forecast about the highest parts of the road.
The heaviest snow is forecast to fall tonight, but tomorrow there will still be showery southerlies, with potential for more snow in Otago.
Farmers are being warned to shelter stock and motorists are urged to take great care on southern roads.
This system also brings a high risk of thunderstorms tonight in Taranaki, Horowhenua Kapiti Coast, the Marlborough Sounds, the far west of Nelson and Buller.
There is also potential for with heavy rain, strong wind gusts and possibly small tornadoes.
In these areas is a moderate risk of severe thunderstorms bringing damaging wind gusts of more than 110 km/h and there is a chance of localised damaging tornadoes.
MetService meteorologist Claire Flynn said from 9pm rain was expected to turn to snow which may accumulate down to 300m, affecting some hill suburbs in Dunedin.
Federated Farmers Otago president Phill Hunt said the cold snap would be an issue for farmers who had "freshly shorn" stock, and they should be moved to more sheltered paddocks.
He said farmers would be keeping a close eye on the weather, in case it deteriorated further, but for most, the southerly blast was more of a gentle reminder that winter was just around the corner.
"Most farmers won't be particularly concerned about the weather.
"Snow doesn't tend to last long at this time of year."
On Tuesday the front moves over the North Island, bringing strong gusts and heavy rain to most areas.
Strong wind warnings have been issued for most country's western areas.
From 7am to 3pm tomorrow Buller and Westland may get severe southerly gales up to 140km/h in exposed places.
Taranaki, Kapiti Coast, Horowhenua, and the Marlborough Sounds can expect gusts of up to 120km/h from 4pm to 11pm tomorrow.
Similarly strong winds are also forecast for Waikato, Auckland and the Coromandel Peninsula from 7pm tomorrow to 1am Wednesday.
The severe weather will start arriving for most areas in the North Island by midday.
"Places like Auckland could really feel the cold front around midday tomorrow," Dyason said.
Auckland is forecast to plummet from 23C today down to 15C by Wednesday.
In the Central North Island the freezing level will drop to 700m tomorrow, with potential snowfall on the Desert Rd.
On Wednesday the storm will start to ease, but cold southerlies will continue for most.
"It will still be quite miserable in a lot of places," Dyason said.
On Thursday the storm will have mostly moved away from the country, however any relief will be short-lived as another system looks set to arrive Friday.
Tips from Energywise to beat the cold snap:
• Clean the filters on your heat pump or dehumidifier so it runs properly. Technical expert Christian Hoerning says heat pump filters should ideally be cleaned every few weeks during winter because they can get clogged.
• If you own a dehumidifier, run it while heating the room. "Dehumidifiers work best in warm rooms and all the electricity they use gets released as heat - so your heater needs to do less work," Hoerning said.
• If the cold draughts are whistling through your home, put low-cost draught stopping tape around windows or doors and tighten up the hinges on doors and windows.
• Avoid lots of moisture in the home over winter, as it increases the risk of mould. Air your home by opening windows and doors a few times a day.
• Dry clothes outside or in a clothes dryer that is vented to the outside – avoid indoor airing racks or clothes dryers that vent into your house. The moisture in the clothes will end up in your home, making it damp.
• Avoid unflued gas heaters which release toxic fumes and make your house damp. Cheap portable electric heaters are safer and cost less to run.
• Check your insulation to make sure it is up to scratch and hasn't been moved over the summer.
