MetService says parts of the South Island above 300 metres altitude can expect up to 100 centimetres of snow. Severe weather warnings are in place from Wednesday night.
Civil Defence is urging people to stock up on essential items and prepare for power cuts that the cold snap could bring.
Snow is falling in Southland and Otago including Dunedin. MetService has also forecast snow for the Canterbury foothills and the Kaikoura Ranges.
MetService spokesperson Daniel Corbett said Christchurch may not get the large amount of snow earlier predicted but the wider Canterbury region can still expect heavy falls. The most disruptive snow will be in areas above 300 metres.
Mr Corbett said the West Coast has a severe wind warning in place for Wednesday night and Thursday with winds of up to 130km/h expected.
Heavy snow in the Canterbury region a year ago cut power to thousands of homes and disrupted travel.
Canterbury Regional Civil Defence group controller Neville Reilly said the major concern is power cuts. People should make sure they have a torch and spare batteries, blankets, non-perishable food and an alternative heating source if the power goes off.
Mr Reilly said Civil Defence has been working with councils around the South Island to organise emergency shelters if needed.
Some 250 Red Cross staff are on stand-by and an emergency centre has been set up in Christchurch. South Island emergency management officer Stephen Manson said the aid organisation has been preparing for the past few days by getting vehicles ready, putting chains on the tyres and stocking up on fuel.
Farmers in Canterbury are being reminded of the 2006 snowfall, which knocked out power and communications in some areas for weeks.
Federated Farmers South Canterbury president Ivon Hurst said the advice from Civil Defence to householders is to have a conventional plug-in telephone and to make sure mobile phones are fully charged. Mr Hurst said people should use mobile text messaging, rather than calls or voice messages.
Canterbury DHB has plan in place
The Canterbury DHB said it has contingency plans in place throughout the region to ensure essential health services keep running.
Chief executive David Meates said in the event of heavy snow making it dangerous for people to leave home, a decision will be made on Thursday morning on whether to postpone outpatient appointments, surgery or other health services.
Dr Meates said people should look after themselves and check in on older people or those with disabilities who live alone. He said if power or heating is lost people should make a plan to stay with friends or family who have heating.
Meals on wheels will continue to be delivered by four-wheel-drive if necessary.
North Otago residents isolated
About 20 people in North Otago remain isolated after a two-day deluge earlier this week caused widespread damage to farms, bridges and roads. Snow is forecast for the area on Wednesday night and more than 40 roads in Waitaki district are still closed.
District council emergency manager Chris Raine said 11 people are cut off in the Gards Road area near Kurow and another 10 people in Danseys Pass where two bridges were washed away.
Mr Raine said Civil Defence has been in touch and they all seem to be doing okay and are getting prepared for at least several more days on their own. Fresh water is being delivered to one farmhouse, but the rest seem to have enough supplies.
The council says it could take up to five days to fix the bridges.