Torrential rain in southeast Australia has caused flooding in the state of Victoria.

Glenelg River, one of the longest in the state, reached major flood levels on Saturday night at the village of Casterton.

The river rose to 6.1 metres and barely dropped during the day on Sunday. About 50 homes were surrounded by water, with several at risk of being inundated.

State Emergency Services Duty Officer Gerry Sheridan told ABC News that residents should avoid the floodwater because of the "unknowns". He said there were a lot of contaminants and toxins in the waters, as well as snakes and vermin.

Venomous tiger snakes had been seen swimming through the streets of the nearby town of Coleraine, where residents of the town were evacuated on Friday night.

A number of people had to be rescued. Emergency services saved one man after his milk tanker became trapped in the early hours of Sunday and another driver in Springfield was rescued on Saturday afternoon. His two dogs were rescued from the roof of his car a few hours later.

Motorists are being urged not to risk driving through floodwaters, which could be deeper than they appear. Monday is expected to be dry in the region, but heavy rain is expected to return on Tuesday and Wednesday.

These torrential downpours could make the flooding worse.