Thousands of properties along the Murray in South Australia could be flooded as forecasts for water flows down the river continue to increase.
The state government says flows are now expected to hit at least 165 gigalitres a day from early December.
There is also a moderate chance of hitting 200 GL and a smaller chance of reaching 220 GL.
That would make it the biggest flood event since 1956 when flows across the border climbed to about 340 GL.
"These new flow predictions raise the risk of flooding in low-lying areas and we are urging people to prepare as best they can," Environment and Water Minister Susan Close said.
"The uncertainties in flow forecasting mean people need to prepare for the worst-case scenario so we get through this event without any risk to life and reduced impact on property."
With water levels continuing to rise, work is underway to repair and improve a series of levees protecting local towns as they face being seriously tested for the first time since the mid-1970s.
Most were built to withstand flows of up to 210 GL.
Plans are also underway to relocate some hospital patients and aged care residents as water levels rise.
Low-lying shacks, homes and other buildings in the Riverland have already been inundated, and more are expected to suffer the same fate.
As the river rises, thousands of properties are now expected to be impacted.
State Emergency Service Chief Officer Chris Beattie said with flows of more than 200 GL a day, about 4250 were at risk, including some farming facilities and industrial premises.
At 220 GL, that would rise to more than 5000.
At about 160 GL, close to 360 would be inundated above the floor level.
"These models have quite a large number of assumptions built into them. So they are indicative only," Mr Beattie said.
"With heightened flows again we would anticipate even more properties to be subject to over-floor flooding."
Local infrastructure has also been hit, with some roads, walking trails and camping grounds already underwater.
Almost all houseboat operators on the river have shut down, with the faster-flowing water proving too dangerous while rising levels will soon close a number of ferry services across the Murray.
Ms Close said there was a concern over forecasts of more rain across large areas of the Murray-Darling Basin next week which could further increase the amount of water in the system.
However, the government said flooding was not expected in SA's Lower Lakes region with the extra water to be managed through the Murray's system of barrages.
