Water flows down the Murray River in South Australia are now expected to hit at least 175 gigalitres a day, increasing the number of properties set to be inundated in low-lying communities.
The peak will hit the SA Riverland early next month, then flow through to the river's lower reaches in the lead-up to Christmas.
The latest forecast has lifted the expected flows from 165GL, but has not raised the worst-case scenario, which has flows at 220GL a day.
Even at the lower flows, up to 4000 properties could be inundated, including shacks, homes, farms and businesses.
About the same number face losing power supplies, probably for several months.
Renmark is considered the community most at risk, with the town's levee system designed to withstand river flows of 210GL a day.
But concerns are also rising for Mannum, east of Adelaide, where work will be done to strengthen its levees amid fears that parts of the town could be flooded.
Premier Peter Malinauskas said the modelling associated with forecasting river flows was "diabolically complicated".
However, he said as the peak drew closer, the forecasts would become more accurate.
"State government agencies are working around the clock with councils, landholders and communities to prepare for the huge volume of water coming across the border," the premier said.
Mr Malinauskas will travel to the Riverland next week to announce a support package for flood-affected locals.
The flood event along the Murray will be the worst since the 1970s.
Some roads in the Riverland are set to be closed, although most ferry services will continue to operate.
Almost all houseboat operators on the river have shut down, with the faster-flowing water proving too dangerous.