The 54-year-old man's motorcycle helmet was found near Gympie on Wednesday and there are fears he was swept away while trying to negotiate the deluge around Cedar Pocket.
The intense rainfall stems from a slow-moving storm system that has battered the region, triggering flash flooding and claiming one life so far.
The body of a 63-year-old woman was found submerged in a vehicle that had been swept into raging stormwaters west of Eumundi on the Sunshine Coast on Wednesday morning.
Up to 600mm of rain fell across the southeast during the day and at least another 300mm is expected on Thursday, with a raft of severe weather warnings issued.
Queensland Fire and Emergency Services Deputy Commissioner Mark Roche SES said the organisation had dealt with more than 240 incidents since 5am on Wednesday, including 19 rescue jobs.
"We've got quite a number of SES volunteers prepped and ready to provide support where needed ... mainly around the southeast corner and on the north coast," he told the Nine Network.
"But we are saying to people they need to take responsibility as well" and not enter flooded areas.
Prime Minister Scott Morrison said it was a tough time for Queensland, adding no one wanted to see any more loss of life.
After weeks of drenching summer rain, many catchments could not cope with the downpour and began spilling on Wednesday and through the early hours of Thursday.
"This has the potential to be a significant rainfall event for southeast Queensland," Premier Annastacia Palaszczuk said on Wednesday.
"Since many catchments are now saturated, there is an increased risk of dangerous and life-threatening flash flooding over the coming days."
The Bureau of Meteorology has issued a general flood warning for southeast Queensland amid signs the weather system will intensify through Thursday and into Friday. Damaging winds are also expected.
The Southeast Coast and the Wide Bay-Burnett region bore the brunt on Wednesday with Burnett, Burrum and Cherwell, Mary, Noosa, Pine, Caboolture, Brisbane, Logan and Albert, Condamine rivers beyond capacity as well as rivers and creeks on the Gold Coast and Sunshine Coast.
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