Communities in three Australian states have been ordered to evacuate as torrential rain brings major flooding.

Parts of the country have received up to four times their average October rainfall in just 24 hours.

At least 500 homes have been flooded, one person has died and another is missing as the disaster unfolds.

Widespread flooding across Australia - driven by a La Niña weather pattern - has killed more than 20 people this year.

Victoria - Australia's second most populous state - has been worst hit this week. Several communities have been ordered to evacuate, including some in the state capital Melbourne.

Floods have swamped roads, forced school closures and cut power to 3,000 houses and businesses.

Premier Daniel Andrews said the number of inundated homes was "absolutely certain to grow", calling it one of the state's worst flood events in decades.

"This has only just started, and it's going to be with us for a while," he told the Australian Broadcasting Corporation.

Barry Webster, who lives in Melbourne's north-west, is one of those whose house has gone underwater.

"I always said I wanted riverfront views, but not like this," he told The Age.

"Going downstairs and seeing the lounge floating… it's a bit surreal, kind of like a movie."

Many areas received massive 24-hour rainfall totals, but the highest was in Strathbogie, north-east of Melbourne.

It received 220mm - more than double the town's average October rainfall, or about a third of London's annual average.

Several rivers have also flooded in Tasmania after up to 400mm of rain fell in some areas in 24 hours. It is unclear how many homes and business have been affected there.

In New South Wales, about 600 people were told to evacuate from the town of Forbes, where about 250 properties and business were expected to flood.

One man died in the state's west earlier this week after his car became submerged in floodwaters.

Rescuers have also been searching for man thought to have been swept away in similar circumstances on Tuesday.

More rain is forecast in the coming weeks, placing strain on already swollen rivers and saturated ground.

Experts say recent flooding in Australia has been worsened by climate change and a La Niña weather phenomenon. In Australia, a La Niña increases the likelihood of rain, cyclones and cooler daytime temperatures.