Christchurch motorists heading across the South Island this weekend are being warned to prepare for heavy rain, severe gales and potentially difficult driving conditions.

MetService has issued an orange strong wind warning for the Canterbury High Country from 1am on Sunday, with west to northwest winds expected to gust up to 120km/h in exposed places.

The warning could affect people travelling between Christchurch and the West Coast, as well as those using high country routes elsewhere in Canterbury.

Orange warnings have also been issued for heavy rain in the ranges of the Buller and Grey districts from Saturday afternoon, and strong winds across southern Southland and Stewart Island from Sunday morning.

A heavy rain watch is in place for the ranges of the Westland District from 9am on Saturday, meaning motorists travelling from Christchurch towards the West Coast should check conditions before leaving.

New Zealand Transport Agency said heavy rain could make roads slippery and increase the risk of surface flooding, slips and fallen debris.

Drivers were advised to reduce their speed, increase following distances, avoid sudden braking and use headlights during the day.

Motorists should not attempt to drive through floodwaters when the depth is unclear, as roads may be damaged or hidden obstacles could be present.

People travelling in campervans, towing vehicles or riding motorcycles were urged to take particular care in exposed areas during strong winds.

NZTA contractors will monitor the state highway network throughout the weekend and respond to slips or other obstructions.