Surging floodwaters broke levees in disaster-hit Australia on Monday to inundate more properties in the southeast, as residents sandbagged homes against the spiraling crisis.

Swollen rivers in the southeastern state of Victoria have created a flood zone measuring an estimated 90 km long and 40 km wide, the State Emergency Service said.

"This area has seen unprecedented flooding," SES spokesman Kevin Monk told AFP. "This is just amazing."

As the floodwaters rushed toward the Murray River, evacuation alerts were issued late Sunday and early Monday for the small communities of Pental Island and Murrabit West, home to about 400 people each.

In an emergency alert the SES said that levees around Murrabit West were failing, warning that the area would be inundated in the next 12 hours.

"They are being flooded now," Monk told AFP. "It's across properties. If they haven't sandbagged them, there may be some impacts on people's housing."

The Victoria floods stem from La Nina-provoked torrential rains which hit the state in mid-January and followed weeks of widespread floods to the north that killed at least 30 people and devastated mining and farming sectors in Queensland.

Prime Minister Julia Gillard again called on companies to boost their donations to the rebuilding effort, with infrastructure repairs and help for businesses and families estimated to cost some A$20 billion ($19.8 billion).

Champion American cyclist Lance Armstrong, who has been in Australia for the Tour Down Under in Adelaide, did his part, leading some 2,500 people on a Queensland Ride Relief fundraiser around Brisbane.