NSW has continued its virus-free streak with no new local cases recorded on Saturday, but dozens of Sydney suburbs remain on alert.
One infection was diagnosed in hotel quarantine.
The news comes as fragments of COVID-19 were detected in the sewage treatments plants at Liverpool and Glenfield.
Hundreds of thousands of residents have since been urged to monitor for symptoms.
The Liverpool plant covers 180,000 residents across Bardia, Hinchinbrook, Hoxton Park, Abbotsbury, Ingleburn, Prestons, Holsworthy, Edmondson Park, Austral, Cecil Park, Cecil Hills, Elizabeth Hills, Bonnyrigg Heights, Edensor Park, Green Valley, Pleasure Point, Casula, Hammondville, Liverpool, Moorebank, Wattle Grove, Miller, Cartwright, Lurnea, Warwick Farm, Chipping Norton, Voyager Point, Macquarie Links, Glenfield, Catherine Field, Gledswood Hills, Varroville, Leppington, West Hoxton, Horningsea Park, Middleton Grange, Len Waters Estate, Carnes Hill, and Denham Court.
“NSW Health urges everyone living or working in these suburbs to monitor for symptoms and get tested and isolate immediately if they appear,” the department said in a statement.
The Glenfield treatment plant, takes in a catchment of more than 160,000 people, across Airds, Ambarvale, Appin, Bardia, Blair Athol, Blairmount, Bow Bowing, Bradbury, Campbelltown, Casula, Claymore, Currans Hill, Eagle Vale, Englorie Park, Eschol Park, Gilead, Glen Alpine, Glenfield, Gregory Hills, Holsworthy, Ingleburn, Kearns, Kentlyn, Leumeah, Long Point, Macquarie Fields, Macquarie Links, Menangle Park, Minto, Raby, Rosemeadow, Ruse, St Andrews, St Helens Park, Varroville and Woodbine.
An alert was issued for the Glenfield catchment just three days ago while the Liverpool plant was last on alert in early January.
The health department said “this likely reflects known recent confirmed cases in these areas” but people should still take caution.
This follows Friday’s alert after COVID-19 was detected in the sewage plants at Camellia and Auburn.