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Forecasters warn a major storm is about to be unleashed across much of the country.
Frigid Antarctic air has left the Ross Ice Shelf and is heading for the South Island.
Bitter weather could hit Canterbury between tomorrow afternoon and Wednesday, when snow flurries to low levels and cold southerly winds may ease.
However, snow and ice are likely to be a problem all week, with heavy frosts likely to stop the snow melting as quickly as it did after last month's fall.
MetService and Blue Skies Weather predict that gales and heavy snow will combine to cause chaos for Canterbury farmers, workers, schools, motorists and travellers over the next few days, with up to 20 centimetres settling close to sea level, 40cm on Banks Peninsula and inland, and more than 50cm above about 200m by Wednesday.
Gale or severe gale south to southeasterly winds are likely to batter the South Island's east coast on Monday.
However, forecasters say it is too early to tell where the worst weather will be felt.
Snow several weeks ago fell in about 18 hours, but this storm may last at least twice that long.
Blue Skies forecaster Tony Trewinnard said the forecast models signalled it could be one of the coldest storms "in living memory", but there was a chance the iciest air might miss the South Island.
"I am a little bit suspicious that the computers may be overestimating the severity of the storm. So, we could end up with as much snow as we had in July, or we could end up with more. We won't know for a day or two yet."
Maximum temperatures would struggle to rise much above 0 degrees Celsius on Monday, with temperatures at 1500m likely to be as low as minus 10C, he said.
MetService spokesman Dan Corbett said the cold air on its way north was "just as cold, if not colder [as in July], so you have got a greater chance of snow".
Christchurch International Airport is preparing for a "worst-case scenario".
Spokeswoman Leeanne Carson-Hughes said the airport was prepared.
"We've got all our equipment out and ready to go; we've rostered all our staff on; we've filled up all our fuel tanks and we've briefed our airlines. We're just ready and waiting for this snow to arrive," Carson-Hughes said.
The Canterbury District Health Board said yesterday that all staff were "considered essential" and were expected to turn up for work.
Chief executive David Meates said managers were co-ordinating with teams to ensure staff could travel safely by four-wheel-drive vehicles. Graders and a gritting team were available to clear hospital sites and contractors had also been organised for Lincoln and Rangiora hospitals.
Meates urged people to look out for older neighbours and relatives.
Civil Defence regional emergency management office manager Jon Mitchell said he was aware some people had not topped up power managers before last month's snowfall. Snow could fall over a wider area this time, meaning people in rural areas also needed to be prepared.
Planning
Ministry of Education
The Ministry said it was up to individual schools to decide if it was appropriate to open or close.
Power Supply
Orion said contingency plans were in place, including standby crews and diesel generators. It was confident the network could cope if householders plugged in extra heaters.
Environment Canterbury
Environment Canterbury said any changes to bus services would be advertised on its website.
CHCH City Council
The council said it would keep residents updated on roads and services.
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