Premier Annastacia Palaszczuk said the 60-year-old was found in the vehicle at Skyring Creek, near Cooroy, on the Sunshine Coast just after 8am on Wednesday.
"This is indeed a tragedy during this particular time," she told parliament.
Swiftwater rescue teams had conducted 23 rescues in the Sunshine Coast and Gympie regions by 9am on Wednesday, after 12 hours of intense rainfall.
Ms Palaszczuk another four rescues were underway as she addressed state parliament.
"Some rescues are ongoing due to challenging and dangerous conditions," she said.
Meanwhile, a train has been derailed at Taveston northeast of Noosa amid the deluge on Wednesday.
The Queensland Fire and Emergency Service has sent emergency alerts to residents in the Gympie, Noosa and Sunshine Coast local government areas warning of flash flooding.
"Residents are being advised that roads and bridges may be impacted," the alerts say.
Queensland Rail said buses were replacing trains on the Sunshine Coast on Wednesday morning due to the severe weather and after a freight train derailed between Traveston and Cooran about 3.20am.
QR acting chief executive Scott Riedel said the driver, who was the only person aboard, was shaken but not injured.
"Queensland Rail will work with the relevant parties to investigate the incident," Mr Riedel told AAP in a statement.
"We thank customers for their patience while we work to recover the site, assess and repair any damage, and monitor weather impacts before we allow services to safely resume."
The Bureau of Meteorology has issued a general flood warning for southeast Queensland after rainfall of 100-408mm were recorded in some areas.
The worst-hit areas have been the Southeast Coast and the Wide Bay-Burnett region, including the Mary River where major floods earlier this year killed three people and damaged dozens of homes and businesses.
The BoM has also issued a generalised flood alert for Brisbane, the Gold Coast, Ipswich, Toowoomba and the Darling Downs as the heavy rain continues.
Premier Palaszczuk said up to a month's rain could fall in some areas over Wednesday and Thursday.
"Since many catchments are now saturated, there is an increased risk of dangerous and life-threatening flash flooding over the coming days," she said.
Meanwhile, the Burnett, Burrum and Cherwell, Mary, Noosa, Pine, Caboolture, Brisbane, Logan and Albert, Condamine rivers are set to break their banks, as well as rivers and creeks on the Gold and Sunshine Coasts.
Seqwater says emergency flood releases from the Somerset and North Pine Dams are possible in the next 48 hours.
Five dams are already spilling, including the Leslie Harrison, Lake Macdonald (Six Mile Creek), Poona, Ewen Maddock, Cooloolabin dams.
The operator said outflows have also increased from the Baroon Pocket, Cedar Pocket and Wappa dams.
"If you are downstream of the dams, stay away from fast-flowing or deep water near waterways and floodplains," Seqwater said in a statement.
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