Images broadcast on Australian media showed homes and roads swamped, areas of land transformed into lakes and a large chunk of a concrete pier floating down a fast-running river.
The waters 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.
"In some parts of southeast Queensland, this is the biggest event that they will see in a number of decades," said state police and emergency services minister Mark Ryan.
"And the rain has not stopped -- in fact, there are some parts where it is intensifying."
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."
'Trapped'
Queensland Premier Annastacia Palaszczuk said the Mary River could rise above 21 metres (69 feet) at Gympie. It may exceed levels last seen there in February 1999, the bureau of meteorology said.
As a precaution, Queensland would be issuing evacuation alerts, notably targeting about 700 residents in the Gympie area, the premier said.
"If we don't do this now people will become isolated and trapped," she warned.
Emergency services had 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.
