As it happened: Victoria records five new COVID-19 cases as Melbourne lockdown restrictions eased; NSW records no new cases as Australian death toll jumps to 875

"That tells us it has basically been shed ... through the shower, washing hands or going to the toilet.

"It's a terrific way to identify where there may have been undetected cases and where cases are persisting and ... where there may be a cluster forming."

Ms Graham said Sydney Water will take advice from NSW Health on whether further testing of positive samples is necessary.

She said the program will become especially useful if clinical testing numbers fall.

"NSW Health can use that to help inform potentially where they might want to pick up more testing of local residents, find out whether or not current controls are working well," Ms Graham said.

Sydney Water laboratories are currently testing raw sewage from plants in Bondi, Malabar, North Head, Cronulla, St Marys, Quakers Hill, Rouse Hill, Penrith, Shellharbour and Winmalee.

Testing began about two weeks ago.

NSW Chief Health Officer Kerry Chant said: “This is a program that will show us where COVID-19 has been. For instance, we would expect to see viral fragments in Sydney sewage where we have consistently had cases in the community or in hotel quarantine.

“However, if we continue to have very few active cases, there is scope for this testing to provide early warning in places without known or recent cases."