A number of factors led to a burst sewage pipe spilling about 450 megalitres of effluent into the Albert River south of Brisbane, the report said.
The figure is 100ML more than what was first reported when the state government launched its own investigation into the leak.
About five megalitres a day spilled from the Gold Coast City Council sewage system into the river from January 11 to April 8.
Corrosion caused the pipe leak, while the Gold Coast council's reliance on the public and another local government to detect spills contributed to why it took so long to identify, the report said.
"The extent of the spill was confirmed on April 12 with a total calculated spill volume of approximately 450 ML," the independent report commissioned by the Gold Coast council said.
The council may face prosecution after the state government announced it would also investigate the "catastrophic failure" back in April.
"We haven't seen a spill of this magnitude in Queensland to my knowledge," the Department of Environment, Science and Innovation said at the time.
The state government probe is ongoing.
The independent report looked at the cause of the leak and the delay in detection.
It said the Gold Coast council had a dependence on the public to report spills, noting "this method of alert is ineffective" for isolated sections such as where the leak occurred.
Higher than average river flows at the time had also reduced the potential for the public to notice the spill, the report said.
A lack of communication from neighbouring Logan City Council about significantly lower inflows at their Beenleigh plant also contributed to the leak detection delay, the report said.
The Gold Coast council said it had adopted the report's recommendations.
"Much of this work is already underway including the development of a four-year critical sewerage pressure main plan, targeted condition assessments of all high-risk mains and reviewing of the specific piping installed during that period," council CEO Tim Baker said.
He said the council was also co-operating with the state government probe.
"We can assure the community that we have taken this incident very seriously," Mr Baker said.
The council will work with the state government and specialist consultants on water quality tests to better understand any environmental impacts, with results expected in August.
Local prawn farms had been forced to stop their operations while the leak was contained and anyone who caught seafood in the river was urged not to consume it.
More than a month after the spill was first reported, the Gold Coast council reopened the river for commercial and recreational fishing.
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