Little more than three weeks out from winter, Adelaide has had its hottest day for May in more than 90 years.

The mercury had soared above 30 degrees Celsius by lunchtime, the hottest temperature recorded for the city in May since 1921.

The reading was more than 10 degrees above the May average and the mercury peaked above 31 by early afternoon.

Temperatures also went into the low 30s across western agricultural areas and the far north of South Australia.

The stretch of warm days is expected to continue until forecast rain at the weekend.