More than 800 firefighters are tackling the blazes, which have been burning for several days in the hills around the city of Adelaide.
Officials say the fires are the worst in the area since the Ash Wednesday bushfires in 1983, which left 75 dead.
Cooler weather may help firefighters tame the flames, which have been fanned by high winds and temperatures.
But South Australia Premier Jay Weatherill said the situation remained critical.
He said 12 homes had been confirmed destroyed in the Adelaide Hills with another 20 also feared lost.
Twenty-two people, mostly firefighters, are said to have suffered minor injuries.
More than 27,000 acres of land has been burned, according to reports.
People are being encouraged not to take any risks, and leave if their houses are in danger.
“Right at this moment, residents in the Adelaide Hills are being confronted by a fire which hasn’t been seen in the hills since the 1983 bushfires of Ash Wednesday,” South Australia fire chief Greg Nettleton said on Saturday.
The 1983 fires left 75 people dead and caused devastation across parts of Victoria and South Australia.
Australia faces such fires every year but environmentalists say global warning is making their occurrence more frequent.
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