Snow is forecast for Christchurch, Banks Peninsula and Canterbury this week, with "major consequences" for farmers, stock and motorists.
Yesterday, flooding saw schools and shops shut and many roads impassable, but the worst may be to come.
Civil Defence is urging Kiwis to stock up on emergency supplies, winter woollies, check their heating and prepare to stay indoors, with "bone-chilling" conditions forecast.
Blue Skies Weather forecaster Tony Trewinnard said the amount of snow forecast for Canterbury on Thursday and Friday was looking "a little more uncertain".
Snow would almost certainly fall on the Canterbury foothills, plains and Banks Peninsula, with up to a metre of snow possible above 400m.
"That will have major consequences for farmers and for stock and for motorists in those areas," he said.
Christchurch would definitely see snow, but the amount and length of the expected snow showers were unclear.
"The latest computer model data have raised the temperatures just a little. From the sea-level to 200m range, it's difficult to make a firm prediction about whether there'll be substantial snow or if it will be mostly rain," he said.
"People like to know is it going to snow or not? Well actually, it's not that simple."
Trewinnard believed Thursday and Friday's snowstorm could be Canterbury's worst since 1992 because of the combination of snow, wind-chill, freezing temperatures and length of bad weather.
Winds could become near gale force (up to 55kmh) across the Canterbury plains from Thursday afternoon to Friday morning and could be near severe gale force (up to 70kmh) on Banks Peninsula.
With temperatures hovering around zero degrees Celsius, the wind-chill could be equivalent to -10C.
"It'll be windy, it'll be bitterly cold. It will be an unpleasant period, unfortunately."
Trewinnard said the weather should be dry on Saturday, but would be cloudy and still very cold, bringing the risk of severe ice on Saturday night.
Civil Defence today issued a list of things people could do to minimise risk to themselves and their families when the big chill hit.
The list included avoiding leaving home unless absolutely necessary when a snow warning is out, and if travel was essential, ensuring you had snow chains, sleeping bags, warm clothing and essential emergency items.
Civil Defence advised that if people were caught in their car or truck in a snowstorm, they should stay in the vehicle, run the engine every 10 minutes to keep warm, open the window a little to avoid carbon monoxide poisoning, and ensure they were visible to rescuers by tying a bright-coloured cloth to the car's radio aerial and keeping the inside light on.
It would be wise for households to have an emergency plan before the storm hit, Civil Defence said.
When a storm struck, people should listen for updates on local radio stations where authorities would be broadcasting advice, it said.
MetService meteorologist Daniel Corbett said the weather would start to change tomorrow when winter would return "with some really angry bark and bite".
By the end of today a southerly change would come across the south of the South Island, and the snow level was expected to drop to about 1000m in Canterbury, Corbett said.
The floodgates from the Antarctic were opening, he said.
"The southerly change works its way through the north on Wednesday, comes through Wellington like a freight train with no driver, and it just keeps going north all the way through to the far north of the North Island," Corbett said.
Significant snow was expected to come with the low, though pinning down exactly where depended on where the low set up, he said.
"One area to flag up right now is probably Mid-Canterbury up to Marlborough,'' he said.
"Farmers and travellers need to be aware, though, on Thursday and Friday."
Weekend weather was set to be better, with the strong southerlies bringing rain easing on Friday.
"By Saturday, the pressure starts to rise from the west, the southerly starts to ease and it dries out, with some overnight frosts down south but improving for the weekend," Corbett said.
Already Christchurch has had well over a month's worth of rain in a deluge that wreaked havoc across parts of the South Island, cutting off Dunedin and causing a deadly landslide north of Motueka.
Wet roads may also have contributed to the death of Oliver Steven Mills, 18, whose car was found upside down in a flooded ditch near Tai Tapu about 10.30am on Sunday.
In Christchurch, more than 90 millimetres of rain fell in two days to bring this month's total rainfall to 108mm. The city typically has 60mm of rain for the whole of June, although in the wettest June on record, in 1995, it got 183mm.
The deluge closed four schools and about 30 roads across Canterbury.
Graceworks Demolition and Recycling managing director Paul King said his staff were busy cleaning up after yesterday's downpour.
Their Mulcocks Rd workplace, near Rangiora, saw floodwaters reach about 35 centimetres in the yards and about 15cm in the sheds.
"We had to stop the timber floating out of the gate. We can't get into one yard because it's so muddy and our main yard has just got debris all over it."
The workers hoped to clear up some of the mess before Thursday's predicted snowstorm hit and shut down their operation again.
King said the business experienced a similar flood a few years ago.
"It was a river through our property. In fact, we actually made a raft and made the best of a bad situation," he said.
BY THE NUMBERS
90: About 90mm of rain fell on Sunday and Monday in Christchurch. The city typically has 60mm of rain for the whole of June.
108: So far this month Christchurch has had 108mm of rain. The wettest June on record was in 1995, when 183mm fell.
30: More than 30 roads across Canterbury were either closed or partially closed as a result of flooding.
4: Weather forced the closure of four schools in the region – Christ the King School, Rangiora High School, Ashley School and Sefton School.
120: Firefighters attended more than 120 incidents across the South Island, including at least 90 in Canterbury.
20: Up to 20cm of snow could fall to sea level in Canterbury this week.
- The Press