Some relief is in sight as Adelaide today reached its 15th consecutive day above 35 degrees Celsius.

The mercury soared to 40.5 degrees in the middle of the afternoon.

Adelaide now holds a capital city heatwave record in Australia, having clearly passed the 10 consecutive days of extreme heat in Perth in February 1988.

Mike Bassanelli, from the weather bureau in Adelaide, says the sweltering heat across South Australia will now start easing.

"The cool air has already begun to cross the Nullarbor coast and even almost at Port Lincoln, so yeah we will see a shallow change come through," he said.

"We should drop that temperature by a bit over 10 degrees."

A maximum of 29 degrees is forecast for Adelaide tomorrow, and a cooler change on Wednesday night.