Snow could affect key highways through Canterbury’s alpine passes and the Mackenzie Basin this week, with drivers warned to prepare for possible closures and delays.

NZ Transport Agency Waka Kotahi said private forecasts supplied to the agency indicated a risk of 10cm of snow affecting State Highways 8 and 80 in the Mackenzie Basin early on Thursday, September 17.

More than 10cm could fall on higher sections of State Highway 73, including Porters Pass and west of Arthur’s Pass village.

The snow risk comes as a cold front is forecast to reach the South Island on Tuesday, bringing heavy rain to western areas already sodden after recent storms.

MetService said warm temperatures across eastern parts of the island were expected to plunge on Wednesday, with snow forecast for higher roads on Wednesday and Thursday.

Conditions could be “favourable for large amounts of snow at higher elevations” later in the week.

NZTA said roading crews would monitor state highways, particularly alpine passes, and would be ready to clear snow or introduce closures and restrictions where needed.

Further south, the cold front and accompanying rain were expected to increase the avalanche risk on State Highway 94 near Milford Sound.

Part of the highway was likely to close from Tuesday evening, with reopening provisionally expected around midday on Wednesday after checks were completed.

A MetService Heavy Rain Watch was also in place for Fiordland from 5pm on Tuesday.

NZTA urged people travelling through affected areas to carry warm clothing, water and food in case they became stuck or needed to wait for conditions to improve.

Drivers should also ensure they had enough fuel or battery range to accommodate any detours.

In heavy rain, the agency advised motorists to reduce their speed, increase following distances, avoid sudden braking and use headlights during the day.

People were also urged to avoid driving through floodwater where the depth was unclear or obstructions could be hidden.