Sewage in Greater Dandenong was released into waterways due to the system being overloaded by heavy storms on 28 February.
During a deluge of up to 48mm – much of which fell in minutes – there was flash-flooding across the council area and greater Melbourne.
Stormwater drains became overloaded, leading to “controlled spills” that flooded into the sewerage system, South East Water service delivery manager Simon Willis said.
During that night, sensors detected that several of “sewer overflow storage assets” were at capacity, he said.
“Our aim is always to protect the environment and the community, however not releasing the water via our Emergency Relief Structures in these conditions would’ve risked the sewerage system ‘backing up’ and potentially spilling into people’s homes.”
Mr Willis said the sewers were designed to withstand “low intensity” wet weather and cater for a “growing population”.
“Extreme rainfall events can overload the stormwater drainage system, causing sewers to overflow with diluted sewer flows which can impact creeks, rivers and bays.
“We greatly appreciate the cooperation of the local community and apologise for any inconvenience this has caused.”
The stormwater drains are managed by local councils or Melbourne Water, Mr Willis said.
City of Greater Dandenong engineering director Paul Kearsley said he understood the sewerage system should be sealed to “prevent odours escaping and stormwater runoff ending up in it”.
“Council does not have any role in the provision or maintenance of South East Water’s sewerage system.
“When the capacity of the system is breached, the water follows an overland flood path towards major tributaries and then ultimately into Port Phillip Bay.
“In the case of any failures in its sewerage system, SEW audits its sewers.”
A Melbourne Water spokesperson said: “There were no controlled or uncontrolled sewage spills during or after the rain event in the Melbourne Water sewerage network.”
Environment Protection Authority Victoria reported poor water quality conditions on Port Phillip’s eastern beaches on 28 and 29 January – due to storm run-off.
On 8 February, water at 36 Port Phillip beaches was reportedly ‘good’.
