Dozens of suburbs in Australia's biggest city have been put on high alert as Covid-19 was detected during surveillance of Sydney's sewage.
Fragments of the virus were found during routine analysis of the wastewater, suggesting there may be undetected cases of coronavirus in the community.
The Covid-19 fragments were found at wastewater treatment plants in North Richmond and West Camden.
Last month there was one positive case of Covid-19 in West Camden, but there have been no recent cases of the virus from the North Richmond area, according to NSW Health.
Dr Richard Broome, acting executive director from Health Protection NSW, said the findings could mean there are undetected cases in the community.
But he also said recovered cases can continue to "shed" the virus for up to four weeks.
"We're calling on people in these catchments to come forward for testing with even the mildest of symptoms such as a runny nose or scratchy throat," he said.
Broome said NSW Health had been working with Sydney Water to analyse wastewater samples since July.
He added there is no evidence Covid-19 can be transmitted through wastewater, and said the virus is deactivated by the treatment process.
The worrying find comes as NSW reports its 11th consecutive day with zero community transmission.
NSW needs 28 consecutive days of no cases of community transmission before Queensland's Premier, Annastacia Palaszczuk, will consider reopening the border to NSW. However, the premier has also said testing rates and sewage analysis would be taken into consideration.
NSW has a target of 8000 people being tested per day. In the 24 hours to 8pm on Monday, only 5385 people were tested. In the 24 hours prior to that, only 4789 people were tested.