Tomorrow and Monday are due to be very unsettled

A "significant" snowstorm has much of Canterbury in its path tomorrow and Monday.

Farmers have been warned to ensure vulnerable stock have shelter from tonight because of strengthening, bitter southeasterly winds, heavy snow down to about 300 metres, sleet and icy rain.

The higher Canterbury Plains, Port Hills and Banks Peninsula are likely to get the most snow.

Blue Skies Weather is warning up to 20 centimetres of snow may settle down to 300 metres, up to 50cm down to 500m and more than a metre of snow above 1000m.

MetService has issued a severe weather watch for heavy rain and snow and will decide this morning if full-scale warnings are needed.

The service's severe weather outlook said there was "high confidence" of heavy snow and heavy rain from the Kaikoura coast to the Waitaki River tomorrow and Monday, with severe southerly gales likely to be a problem for Banks Peninsula.

Spokesman Bob McDavitt said the ingredients for heavy snow appeared to be coming together in the right order.

High-level moisture would be arriving from the Tasman Sea in an area of extremely strong winds, helping the low-pressure system to deepen as cold southerly winds moved up the South Island.

"It's not yet clear how much snow will fall, but this could turn out to be a significant snow event for Canterbury and Marlborough, if the low stalls on top of you.

"This outbreak is likely to produce challenging travelling conditions and affect vulnerable livestock."

Blue Skies forecaster Tony Trewinnard said heavy rain would also cause problems.

"Rainfalls over the next few days will be significant. Overall, around 100 millimetres is expected from Saturday afternoon to Monday afternoon, with the heaviest falls likely to be in Mid-Canterbury.

"One hundred millimetres in 48 hours is not to be sneezed at, especially considering how wet the ground still is."

- © Fairfax NZ News