An extreme bushfire risk has been declared for the Adelaide Hills with temperatures expected to soar to 40C and go even higher in some South Australian regional centres.

The Country Fire Service has issued severe bushfire ratings for six other districts on Friday while a severe heatwave has been declared for the northwest pastoral region.

The Bureau of Meteorology says Adelaide should hit 40C with the mercury to climb to 46C in some outback towns including Oak Valley and Nullarbor, on the west coast.

Similarly hot conditions will continue into Saturday, with Adelaide to have a top of 38C, before a cooler, southerly change pushes through.