Gold Coast council may still face prosecution, with the Queensland government continuing its own investigation into the "catastrophic failure".
About five megalitres a day spilled from the council sewage system into the Albert River, south of Brisbane, for months after corrosion caused a pipe leak.
About 450 megalitres of effluent - the equivalent of 180 Olympic-sized swimming pools - leaked into the waterway from January 11 to April 8.
The council said an independent study it commissioned into the spill's environmental impact found the waterway recovered quickly.
The Griffith University study found significant rainfall flushed out the waterway during the spill, the council said.
A state government department is conducting its own investigation into the spill. (HANDOUT/DEPARTMENT OF ENVIRONMENT SCIENCE AND INNOVATION)
Higher than average flows had reduced the potential to notice the spill before it was finally detected after initially being reported by a member of the public.
"The advice we received at the time was that the significant flows were likely to have diluted the spill, delaying its detection and also lessening environmental impacts - it is a relief this is the case," acting council CEO Alisha Swain said in a statement.
The study looked at oxygen levels, nutrient concentration, accumulation of metals and per- and polyfluorinated alkyl substances or PFAS, known as forever chemicals.
The council said the study found the waterway had recovered to normal levels in "all key areas" and there was no evidence of long-term environmental impacts.
However, it noted low dissolved oxygen in the river due to the spill would have impacted some animals.
"It is likely immobile animals like molluscs were impacted. Fish would have moved away," the council said.
The Department of Science, Environment and Innovation said the independent study's findings would form part of its ongoing probe.
When the spill was detected in April, the department said the investigation could lead to the council being prosecuted for "some fairly serious offences".
"We have received a copy of the summary of findings ... commissioned by the City of Gold Coast Council to investigate potential long-term environmental impacts from the Albert River sewage spill," the department said in a statement.
"We are reviewing the findings, and the report will form part of our formal investigation."
Gold Coast council is already working on recommendations from another independent report that looked at the leak's cause and delay in detection.
It found the council's reliance on the public and another local government to detect spills contributed to why it took so long to identify the leak.
The council said it would continue to co-operate with the state government probe.
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