NSW Chief Health Officer Dr Kerry Chant said a recently recovered case who was infected elsewhere is in the area, and people who are not infectious can still shed the virus.

"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 sewage tests would be repeated.

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Recovered people may shed for several weeks, but the public is being urged to check for symptoms and get tested if needed.

"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."

It's the first time COVID-19 particles have been detected in sewage surveillance programs across the Armidale and Tamworth regions, which have now been occurring weekly for almost a year.

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.

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.

NSWHealth said it is not well understood how long the virus survives in sewage, and it depends on the conditions in the sewer.

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