A BUSHFIRE is burning on South Australia's Yorke Peninsula as temperatures across the state surge above 40C.

The Country Fire Service said the blaze is burning between the towns of Ardrossan and Maitland, and people in the area have been urged to activate their bushfire survival plans.

The CFS has also expressed concern that the extreme conditions could revive a bushfire still burning within control lines in the Clare Valley.

"The greatest chance of fire escape is in the afternoon, which may put the settlements of Watervale, Leasingham and Auburn at risk," the CFS said.

The service said today's extreme conditions would test all its resources ahead of a milder change expected in coastal areas later in the day.

"Everything is super dry, ready to burn," spokesman Malim Watts said.