Heavy snow could disrupt travel, damage trees and bring down powerlines across parts of the Canterbury High Country as a significant winter storm moves over the South Island.

MetService has updated its watches and warnings, with heavy snow possible across the Canterbury High Country south of the Rangitata River and parts of Otago.

A Heavy Snow Watch is in place from midnight Wednesday until 9am Friday.

Periods of heavy snow are expected above 800 metres, where accumulations could approach warning criteria. Lesser amounts could fall as low as 400 metres.

The conditions could affect alpine roads and passes, while farmers have been warned the cold weather may cause stress to livestock.

Rain is forecast to spread north across the Canterbury Plains during Wednesday, becoming heavy at times. It is expected to fall as snow above about 700 metres, with snow levels dropping further in inland parts of the region.

Christchurch can expect rain to develop during the afternoon, with heavier falls possible into the evening as cooler southerly winds spread across the region.

The heaviest conditions are expected through the Mackenzie Country and southern Canterbury, where travel disruption, difficult driving conditions and road closures are possible.

On the West Coast, an Orange Heavy Rain Warning has now been issued for Grey and Buller south of Westport.

Westland is forecast to receive between 300mm and 450mm of rain about the ranges, with 150mm to 200mm possible nearer the coast. Peak rainfall rates of 20mm to 30mm an hour are possible during the most intense periods.

MetService said the prolonged rain could cause streams and rivers to rise rapidly, while slips, surface flooding and hazardous driving conditions were also possible.

The severe weather is being driven by a slow-moving low and associated fronts, bringing warm, moisture-laden air from the north into contact with colder air over the lower South Island.

People planning to travel through inland Canterbury or across alpine routes are being urged to check road conditions and the latest MetService warnings before setting out.