Motorists are being urged to take extra care on two major South Island routes due to worsening winter conditions.

Snow and ice have affected State Highway 80 between Lake Pukaki and Aoraki/Mount Cook, and State Highway 8 between Twizel and Fairlie.

NZTA issued the warnings on Wednesday night, with no detour routes available and no clear timeframe for when conditions will improve.

Drivers are advised to slow down and stay alert, as the warnings remain in place until further notice.