The positive Covid-19 results found in sewage could also be from recently recovered cases who can continue to shed fragments of the virus weeks after recovering.

Australian health authorities are warning that Covid-19 could be going undetected in New South Wales, after fragments of the virus were detected in a third sewage plant in two days.
Fragments of the virus that causes Covid-19 have been found in the sewage system at Burwood Beach Treatment Plant from a sample taken on Wednesday.
The plant has a catchment of around 225,000 people and takes in Newcastle and the surrounding suburbs of Dudley, Charlestown, Jesmond, Lambton, New Lambton, Mayfield, Elermore Vale, Wallsend, Kotara, Garden Suburb, Adamstown Heights, Kahibah, Highfields, Merewether, Waratah West, Georgetown and Carrington.
It follows similar health alerts issued Saturday for the sewage systems at Allambie Heights on Sydney’s Northern Beaches and Merimbula on the NSW south coast.
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These plants take in 83,400 people in the Northern Head catchment and 15,000 people on the South Coast.
Those samples were taken on April 22 and April 21 respectively.
NSW Health noted that the positive results could be from recently recovered cases of Covid-19, who can continue to shed fragments of the virus for several weeks after recovering.
“However, NSW Health is concerned that they could signal undetected cases in the community, and asks people in these areas to be alert for any symptoms that could signal Covid19,” they wrote in a statement.
There are no new locally-acquired Covid-19 cases detected in NSW on Sunday and only one in hotel quarantine, with 7,706 tests conducted Saturday.
One person remains critically ill in ICU and is on a ventilator.
In the state, there are 87 people with the virus. Most are not in hospital.
This story was originally published on Nine News and is republished with permission.