In short:
A report finds the Gold Coast City Council's reliance on the public and a neighbouring council to detect sewage leaks contributed to the worst spill in Queensland's history.
The equivalent of about 180 Olympic swimming pools worth of sewage spilled into the Albert River at Yatala between January and April.
What's next:
The Department of Environment, Science and Innovation is still investigating with findings expected later this year.
The worst sewage spill in Queensland history was caused by corrosion and a local government’s over-reliance on the public and another council to detect leaks, an independent investigation has found.
The equivalent of about 180 Olympic swimming pools of sewage spilled into the Albert River at Yatala, 40 kilometres south of Brisbane, between January and April before the Gold Coast City Council (GCCC) detected the leak.
The council repaired the leak within four days once it was discovered.
In April, the Department of Environment, Science and Innovation (DESI) said the "catastrophic spill" was under investigation and labelled the GCCC as the "polluter".
At the time, DESI executive director Brad Wirth said he had not seen a spill of such scale in Queensland.
That investigation is ongoing.
The GCCC commissioned an independent report by AECOM Australia to investigate the cause of the spill and the delay in it being detected.
The GCCC paid $150,000 for the AECOM report, which was released to the public today.
The report shows the total volume of the spill is now estimated at 450 megalitres (ML), 100ML more than originally reported.
The report also pinpointed the day the leak from a burst sewage pipe began as January 11, by analysing changes in waterflow to the neighbouring Logan City Council (LCC) catchment.
The report said the delay in detecting the leak could be attributed to GCCC's dependence on the public to report spills and the higher-than-average river flows during the period making it difficult for river users to notice.
The burst pipe is owned and operated by GCCC but flows to the Beenleigh Wastewater Treatment Plant, run by the LCC.
The report said a lack of advice from the LCC about reduced inflows at the Beenleigh plant for the duration of the spill contributed to the delay in GCCC detecting it.
A GCCC spokesperson said the council took full responsibility for the spill, and that the independent report differed in scope to DESI's current investigation.
AECOM Australia declined to comment.
Technology to be improved
City of Gold Coast CEO Tim Baker said the investigation determined the break in the pipe was due to corrosion most likely caused by "aggressive" soil or groundwater.
He said the omission of a specified wrapping for the pipe when it was installed in 2005 was also a contributing factor.
The GCCC said it had adopted all recommendations of the report including growing its asset management team and reviewing asset management strategy and undertaking planning for critical sewage rising mains.
It said it would revise the arrangement with Logan City Council, review the condition of pipes installed at the same time as the corroded one, and improve monitoring technology.
He said the city continued to cooperate with DESI's investigation into the incident.
Logan City Council mayor Jon Raven said the LCC information about the water network is shared in real time and live to the GCCC.
"It's a system that shares (the information) live, so that the exact same thing that we see in our situation room for our water network, they have the identical data at the same time in the same way," he said.
"We don't have enough time and we're not paid to monitor their network, we just provide them the data so they can do their jobs.
"It's literally like saying, 'I didn't notice my house was burning down, why didn't my neighbour tell me?'"
He said the LCC was also cooperating with the DESI investigation.
"We'll wait to see their report rather than relying on the puff piece that Gold Coast is paid for," he said.
He said the Beenleigh Wastewater Treatment Plan had alarm notifications for disruptions and changes in the LCC's water network.
"If (the spill) happened in Logan, we'd know straight away," he said.
"When it's in someone else's sewer network, they pay for us to manage their sewer but they don't pay us to do the job of monitoring the network for them."