The entire region was awash on Saturday afternoon and being pummelled by multiple severe thunderstorms from a low-pressure trough which has been sitting over the state's southeast for five days.
The body of a 37-year-old Goomboorian man who went missing in floodwaters near Gympie on Friday night was found by police divers on Saturday, taking the state's death toll to five.
Gympie Council has started evacuating 700 people living along the swollen Mary River, which will rise above its highest level in 23 years in the next 24 hours.
Residents in impacted areas at Southside and on the hospital side of the river were urged to evacuate immediately and seek higher ground.
Flood sirens were sounded in Grantham warning townspeople in low-lying areas to flee to higher ground.
The Lockyer Valley Council later encouraged residents to register with the Red Cross's Register.Find.Reunite service.
"Council is currently fielding calls from concerned family members, seeking to locate loved ones who may be staying in Places of Shelter, or who do not have access to their phones," a statement said.
Moreton Bay residents have also been urged to get ready to flee with "very dangerous" flooding already "above major levels".
Logan Mayor Darren Power has warned floodwaters could cut off parts of Logan Village, Buccan, Jimboomba, Glenlogan, Chambers Flat, Carbrook, Kairabah and Bethania in coming days.
The Indigenous community of Cherbourg, west of Gympie, which will soon be cut off, while there is major flooding at Tewantin near Noosa, and Picnic Point and Dunethin Rock on the Sunshine Coast.
Beaudesert, south of Brisbane, and Maryborough, downstream from Gympie, are also set to be inundated by major floods.
Among the five people to have died from the floods was SES volunteer Merryl Dray who was killed when the car she was in with three other volunteers was swept off a road en-route to a rescue in the Lockyer Valley on Friday night.
Queensland Fire and Emergency Services Commissioner Greg Leach described the 62-year-old's death as a "tragic loss".
"Merryl was a much-loved member of the Lowood SES Group where she volunteered more than 520 hours of her time over the last four-and-a-half years," he said.
More than 70cm of rain fell at Mount Glorious in the 21 hours to 6.30am on Saturday, the Bureau of Meteorology said, with 24-hour downpours of more than half a metre recorded in many other places.
"This is something that is completely unpredictable, in terms of this big rain event occurring over the entirety of southeast Queensland," Premier Annastacia Palaszczuk said.
Deputy Police Commissioner Steve Gollschewski urged people not to leave home unless they absolutely had to amid warnings of possible "dangerous and life-threatening" flash floods.
"We do not want to lose any more people, so please, please, if it's flooded, forget it," he said.
Ipswich is set for its worst flood in a decade with the Bremer River predicted to rise above its 2013 peak of 13.9m on Sunday.
Multiple Brisbane suburbs including Moggill, Jindalee, West End and Capalaba were partially flooded on Saturday, with minor flooding possible in the CBD on Sunday.
Dozens of roads are cut, including the Bruce Highway north of Brisbane and the Warrego Highway west of the city.
Train services north and west of Brisbane are set to be suspended for days, while Brisbane River ferry services have also been halted indefinitely due to dangerous debris in the water.
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