Hundreds of people have called for help in Victoria and South Australia after heavy rain and storms lashed the states as the wet summer continues across Australia's east.
Swathes of Victoria have been put on notice to expect rainfall totals as high as 150mm to 200mm as thunderstorms sweep across the state.
Victorian State Emergency Service personnel have already responded to more than 110 requests for help with at least 53 flood-related rescues since 6am Sunday.
Some areas in the state's north have recorded more than 50mm of rain since 9am.
South Australian and southern NSW areas are also in the firing line of the major weather event.
A severe weather warning was in place for much of southwest NSW and western Victoria on Sunday evening, with damaging rain and wind and flash flooding possible.
The SA State Emergency Service warned of the potential for heavy rain, strong winds, hail and lightning as thunderstorms swept across the state on Saturday night before moving over eastern border districts from Sunday morning.
The Barrier Highway was closed in both directions between Broken Hill and Cockburn on the South Australian border on Sunday due to flooding about 50km west of Broken Hill.
The SES had responded to more than 90 calls for help on Sunday after fast-flowing floodwaters blocked roads in the Flinders Ranges.
Storms knocked out Coober Pedy Hospital's power supply but the SES said the hospital was functioning normally on backup power.
NSW SES spokesman Brett Koschel said three storm crews and two flood rescue teams had been brought to the state's south from elsewhere to assist if necessary.
"In the last 24 hours we've been making preparations for the forecast weather," he said.
"We've had some community members out there preparing their properties, and had those requests for sandbags so they've been able to prepare their properties as best they can."
Victorian SES Commissioner Rick Nugent said flash flooding was highly likely along many of the state's already sodden rivers and creeks following recent rain.
Residents of flood-prone areas, as well as campers and holiday-makers in caravan parks, were urged to prepare and be on the lookout for emergency flood warnings.
"Falling tree branches and flash floods are the highest risk," Mr Nugent told reporters.
"Please don't drive through floodwaters - you're driving a car, not a boat."
The storms were expected to develop in Victoria's west on Sunday before moving through central, north central and eastern parts of the state into Monday.
Some areas in Victoria's Mallee and Wimmera districts could record up to 60mm of rain in less than an hour, meteorologist Michael Efron said.
"The amount of moisture across the state at the moment is incredible," he said.
"It's what you would normally see in somewhere like Queensland."
SES crews established sandbag collection points at high-risk locations such as Bendigo, Castlemaine, Campbells Creek, Heathcote and Wedderburn on Sunday.
