SA bushfire still to be contained

A bushfire south of Adelaide continues to burn freely with back-burning operations planned in a bid to bring it under control.

The fire at Yundi on the Fleurieu Peninsula is burning south through scrub.

The Country Fire Service says the blaze is not controlled and conditions in the area are continually changing.

It has destroyed about 55 hectares and is being fought by about more than 110 firefighters as well as water-bombing aircraft.

Fire crews will conduct major back burning in the area on Wednesday night in an effort to contain the southern flank.

The CFS has urged residents to activate their bushfire survival plans but only leave if it is safe to do so.

The blaze was sparked on Tuesday as temperatures climbed into the 30s across SA and then into the 40s in some centres on Wednesday.

In Adelaide the mercury hit 38.8C at 3.30pm.

A milder change was forecast to move across the state on Thursday.