Diane Robertson, a member of Aura Apartments' body corporate, on Monday told the Queensland Floods Commission of Inquiry the building's design contributed to the damage it sustained in floods last summer.
She said the building, in the inner-city suburb of West End, had pipes designed to let outside water into the basement because the walls were not strong enough to withstand hydrostatic pressure.
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She said much of the building's vital infrastructure, including the electricity supply, was located in the basement.
She said she had been told that the building's plumbing system was designed so that effluent would flush straight into floodwater during a flood.
"Dirty water had come up from inside the toilet and overtopped the bowl," Ms Robertson said, adding that she did not know the source of the sewage.
Ms Robertson raised concerns about the surrounding precinct's transition to medium density residential.
"By approving a 12-storey building in the next street, the Brisbane City Council is creating an area that is more high density," she said.
She said the developers of Waters Edge Apartments had also been given permission to lower the river bank.
"It is from this location that Brisbane River overtopped the bank and contributed to the flooding of Aura Apartments."
The hearing in Brisbane continues.
