Australian police confirmed the toll after a 26ft wall of water - described as an "instant inland tsunami" - inundated the city late on Monday.
The waters swept away cars, houses and pedestrians, killing at least two children and leaving many residents stranded on rooftops for hours.
Unconfirmed reports claim that 50 of the reported missing have been found alive, but 22 people are still unaccounted for.
8 Dead Many Missing in Queensland Floods 2011
The Queensland Premier, Anna Bligh, has described the latest deluge as "without a doubt our darkest hour of the last two weeks".
The flash flood was prompted by more heavy rains falling on the already-saturated state and officials say there was little warning of the surge.
The waters receded almost as quickly as they appeared in the city, which has a population of 90,000 and is located 80 miles west of the state capital Brisbane.
But they soon spread to other smaller communities, inundating the town of Grantham and other rural areas.
8 Dead Many Missing in Queensland Floods 2011
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