A flood watch has been issued for parts of the Adelaide Hills as a complex low pressure system brings heavy rain to the region.
The Bureau of Meteorology says the system will hit the Mt Lofty Ranges from Thursday afternoon into Friday morning.
A severe weather warning is also in place for damaging winds and heavy rainfall for large parts of southern SA including the west coast, the Eyre Peninsula, and the southeast.
Widespread falls of up to 80mm are possible, with totals likely to hit 100mm in some areas.
The bureau says flooding in the Fleurieu Peninsula, Torrens and Onkaparinga rivers and metropolitan rivers and creeks is possible from late Thursday evening.
Flooding in upper Onkaparinga and upper Torrens rivers may lead to road closures.
The warnings have prompted the State Emergency Service to upgrade its alert level to a more active footing.
It is also making sandbags available to local residents across the hills.
State Duty Officer Robert Charlton said the SES was well advanced in its planning for severe conditions.
"We are all very grateful for our fantastic volunteers who will be leading the response, if required,'' he said.
"There has already been an early report of wind gusts over 100km/h in one area and we are preparing for increased requests for assistance from the public throughout the day and evening.''