Queensland Police on Monday said a 59-year-old man died at Fitzgibbon, in Brisbane's north, on Sunday afternoon.
He was trying to cross Cabbage Tree Creek on foot when he was swept away before being pinned against a fence.
Two people raised the alarm and tried to give the man CPR, but he died at the scene.
Queensland Fire and Emergency Services warned life-threatening flash flooding is occurring in the Tallebudgera and Currumbin valleys on the southern Gold Coast on Monday morning.
"Shelter in place unless it is unsafe to do so. Access to area is cut and inaccessible in multiple places," QFES said in an alert.
QFES have made more than 100 swift-water rescues amid 8000 calls for help in the Brisbane, Ipswich, Sunshine Coast and Gold Coast since 3pm on Sunday
The search continues for a sole sailor, aged in his 70s, who fell overboard from his vessel near the mouth of Breakfast Creek in Brisbane on Saturday afternoon.
Multiple emergency flood alerts are in place for about 140 city suburbs, with 2145 homes and 2356 businesses set to be fully submerged by a 4m flood peak on Monday morning's high tide.
Another 10,827 properties will be partially flooded above the floorboards as waters rise to less than half a metre below the 4.46m flood level reached in 2011.
Brisbane council is warning residents along the river to prepare to evacuate, including in Brisbane CBD, Newstead, West End, New Farm, Milton, St Lucia, Bulimba, South Brisbane, Toowong and Rocklea.
Brisbane Lord Mayor Adrian Schrinner said the floods are "very different" to 2011 because the rain pummelled the region for five days.
"We've seen incredible amounts of rain," he told ABC Radio on Monday.
"With 2011 in some ways it was a dry flood, the rain had stopped well in advance and then we had several days notice that the flood was coming.
"It is very different this time. It came very quickly and the rain was so strong, so it is a different event."
Major flood alerts have also been issued for rivers in Gympie, Maryborough, Sunshine Coast, Noosa, Moreton Bay, Ipswich, the Lockyer Valley, Toowoomba, Darling Downs and the Gold Coast.
The state government has told people to work from home and closed all schools in the southeast's 13 local government areas on Monday.
Transport Minister Mark Bailey says major roads including the Gateway Motorway, Bruce and Warrego Highways are cut and trains and ferry services across Brisbane have been halted.
He urged people to stay at home.
"There's still a lot of localised flooding all through southeast Queensland, including the Sunshine Coast," he told ABC Radio on Monday.
"So people need to be careful still make safety their priority.
"We're going to have localised flooding in a lot of areas for a couple of days yet."
There are unconfirmed reports of looting in the Ipswich area and Police Commissioner Katarina Carroll said officers are investigating.
Prime Minister Scott Morrison was due to give a speech in Brisbane on Monday to the Queensland Media Club but the event has been postponed due to the disaster.
After smashing the southeast of the state, the severe weather is heading to the Gold Coast and NSW northern rivers district.
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