More than 30cm of snow could affect South Canterbury highways this week as a significant winter storm moves across the South Island.

NZ Transport Agency said private forecasting suggested more than 30cm of snow could affect State Highways 8 and 80 in the Mackenzie Basin and South Canterbury on Thursday.

Further north, up to 20cm could fall on State Highway 73 above 700 metres between Christchurch and the West Coast.

NZTA said snow accumulations at those levels were likely to impede travel and sections of highway could be closed or placed under restrictions to prevent vehicles becoming trapped.

MetService said a complex low and frontal system would spread onto the South Island from the Tasman Sea on Wednesday, bringing prolonged heavy rain to western regions and possible heavy snow to inland parts of Otago and South Canterbury.

There was moderate confidence that snow could reach warning levels above about 700 metres late on Wednesday across Central Otago, the Queenstown Lakes District and north to the Mackenzie Basin.

The snow risk is forecast to lower on Thursday, with moderate confidence of warning amounts above about 400 metres from Central Otago and the Queenstown Lakes District north to inland South Canterbury.

Moderate confidence represents a 30 to 60 percent chance that warning thresholds will be met or exceeded.

Heavy rain could affect lower elevations, while MetService had low confidence that warning amounts of rain could reach the Canterbury and Otago headwaters on Wednesday and Thursday.

Christchurch was not identified as facing warning level rain, snow or wind, but motorists travelling between the city and the West Coast could encounter significant snow on State Highway 73.

NZTA said road crews would work around the clock to clear snow and other obstructions from affected highways.

People planning to travel through inland areas or alpine passes were urged to monitor weather forecasts and check the NZTA Journey Planner before leaving.

Motorists driving through snow, ice or heavy rain were advised to reduce their speed, increase following distances and avoid sudden braking or turning movements.

Drivers should also carry warm clothing, food and water in case they encounter delays, road closures or detours.

MetService had not issued a severe weather watch or warning specifically for Canterbury but said further alerts would be added as the storm approached.