NSW Health has detected fragments of the COVID-19 virus at the sewage treatment plant in Armidale.
Chief Health Officer Dr Kerry Chant said a recently recovered case who was infected elsewhere is in the area.
"We are aware of someone who is there," Dr Chant said.
"That person is no longer infectious. We have, and we are following up a contact that was associated - a close contact that is also residing in Armidale who had previously tested negative, but we are testing them again."
Dr Chant said the sewerage tests would be repeated. Recovered people may shed for several weeks, but the public is being urged to check for symptoms.
"We have some plausible source for that positive detection in Armidale, but we need to be very sure because that person was residing there for a little period of time and hadn't previously detected," Dr Chant said.
"For the abundance of caution, a call out to the Armidale community. We want you to come out and get tested."
The NSW Sewage Surveillance Program tests untreated sewage for fragments of the COVID-19 virus at more than 60 sewage treatment plants across NSW to provide data to support NSW Health's COVID-19 response.
Fragments of the virus that causes COVID-19 can enter the sewage through an infected person's faeces and when washed off hands and bodies via sinks and showers.
NSWHealth said it is not well understood how long the virus survives in sewage, and it depends on the conditions in the sewer, such as temperature, presence of other microorganisms, and the amount of organic matter but it is not not expected to remain infectious in sewage for a long period.
Finding traces of the virus does not mean there is a person with COVID-19 in the area.
People who are recently recovered from COVID-19 can sometimes continue to shed virus fragments into the sewerage system for several weeks even after they are no longer infectious. It could also mean that a person with COVID-19 might have visited the community and has since left the area.
However, according to the NSW Health website, it could mean there have been one or more people who are infectious with COVID-19 in the catchment area.
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