Huge downpours unleashed decades-high floods in eastern Australia on Saturday, killing two people overnight as the rising waters inundated homes and roads, and swept away cars.

Authorities in the eastern state of Queensland issued 11 emergency alerts in 24 hours as the overall death toll from days of flooding rose to four, with two more people missing.

"In some parts of southeast Queensland, this is the biggest event that they will see in a number of decades," warned the state's police and emergency services minister, Mark Ryan.

"And the rain has not stopped -- in fact there are some parts where it is intensifying."

Images broadcast on Australian media showed homes surrounded by water, roads cut off, areas of land transformed into lakes, and a large chunk of a concrete pier floating down a fast-running river.

The water snatched the car of a team of four emergency services workers who were heading to rescue a family from their flooded home, said state police disaster coordinator Steve Gollschewski.

"The vehicle in which they were travelling was swept off the road into floodwaters. Three of our members were rescued. One of those members is deceased," Gollschewski told a news conference.

Elsewhere, another man's body was found overnight after he was apparently caught up in the floods, he added.

Queensland police said they were also searching for two other people reported missing.

- 'Life-threatening' -
Emergency services responded to more than 1,800 calls for help in 24 hours in southeast Queensland, officials said. 

More than 250 people were in evacuation centres as of Saturday morning, with more than 7,700 power outages in the area.

The Queensland town of Gympie was facing a "hell of a lot of water" as the Mary River rose, Gympie Mayor Glen Hartwig was quoted as saying by national broadcaster ABC.

In the town's Royal Hotel, "without a submarine or a snorkel, you will not be having a beer," Hartwig said. 

"It's floor-to-ceiling and over the roof in some areas."

Rainfall in some areas of the state had exceeded records going back decades, said senior Queensland meteorologist David Grant.

He predicted further rain on Sunday as the weather system moved away more slowly than anticipated.

"There is now going to be an increased risk of dangerous and life-threatening flash flooding, and even the potential for localised landslides," he told a news conference.

Heavy rain also hit the state capital Brisbane.

"Essentially we've seen a month's worth of rainfall fall in one day for just Brisbane alone," Grant said.