South Australian Premier Peter Malinauskas has warned residents to prepare for the risks that lie in the days ahead.
A severe weather alert is in place for the state's western coast and west pastoral and Eyre Peninsula districts, with six-hourly totals of 30-70 millimetres likely and isolated totals of 120mm possible.
In the southeast, which has barely had a drop of rain all year, local falls up to 100mm are on the cards.
Mr Malinauskas said the system's unpredictability meant the entire state was on alert.
"When we start to see rain events north of 50, 60, 80ml, that starts to represent the sort of conditions where we will see flash flooding," he said.
"If we see that volume of rain in a short period of time, then we should anticipate flash-flooding events throughout the state."
Heavy rain and severe thunderstorms potentially leading to flash-flooding has been forecast for parts of the north, west and southeast.
Mr Malinauskas said while the rain was poorly timed for South Australia's famed viticulture industry, it would also bring relief to many.
"For pastoralists and the grain-growing community, much of this rain will be very welcome indeed," he said.
Senior Bureau of Meteorology forecaster Jon Fischer said the slow-moving tropical low - which had sat over the Simpson Desert in the southeast Northern Territory for a week - was highly unusual.
"We've all seen images of roads being washed away, impacts to the rail line and usually dry creeks turning into torrents," Mr Fischer said.
The system is forecast to move southeast over the course of the weekend, with a flood watch issued for all catchments across the state.
Adelaide was expected to feel the effects late on Sunday and overnight into Monday morning.
Flood warnings remain for large parts of central Australia, with much of the water flowing into Kati Thanda-Lake Eyre.
The Northern Territory's Barkly and Simpson districts could see up to 80mm of rain on Saturday, while localised intense falls up to 110mm could result in life-threatening and dangerous flash-flooding.
"These conditions also threaten cattle and other livestock," the weather bureau warned.
Conditions are expected to ease across the territory's southeast by Saturday evening.
By next week, the system is expected to push into western Victoria, with falls up to 150mm anticipated.
Meanwhile swimmers and surfers hare being warned of increased bull shark activity in Sydney Harbour and estuaries along the NSW coast after recent heavy rain.
Rich run-off can attract baitfish and, in turn, bull sharks.
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