Fragments of COVID-19 were detected at the Lilydale wastewater treatment plant, the inlet to the Werribee wastewater.
An urgent health alert has been issued to residents in Tarneit, Werribee and Hoppers Crossing, who have been advised to get tested if coronavirus symptoms appear.
There are currently 24 suburbs on alert. Residents in Lilydale, Chirnside Park, Coldstream, Kalorama, Olinda, Montrose, Mt Dandenong, Mt Evelyn, Yarra Glen and Yering are among those asked to monitor for symptoms. The catchment includes some or all of Taylors Hill, Plumpton, Hillside, Sydenham, Delahey, Caroline Springs, Burnside Heights, Kings Park, Albanvale, Burnside and Deer Park.
Anyone in the area from 20-22 February should get tested.
READ MORE: $452 million in response to 150 recommendations in Aged Care report
The fragments were found in samples collected on 25 February despite no recording of local transmissions.
On Monday afternoon, the Department of Health and Human Services (DHHS) said independent testing confirmed the results.
A DHHS statement reads: “Anyone who has any symptoms of Covid-19 is urged to get tested, especially if you live in or have visited these areas from February 23 2021.”
Health authorities have expanded testing sites in the area.