Finding the source of repeated detections of COVID in one town's sewage is "a needle in a haystack moment", NSW Deputy Premier John Barilaro says.
NSW Health and the Bega Valley Shire Council confirmed on Wednesday that a sample from Merimbula's sewage treatment plant again tested positive to fragments of the COVID virus - the third time in just over a week.
Samples from the South Coast towns of Bega, Bermagui and Eden did not return a detection of the virus during the same period.
Bega Valley Shire Mayor Russell Fitzpatrick said while there were still no known cases in the Bega Valley, the persistent presence of fragments continues to signal the possibility of undetected cases in the community.
"The detection of fragments of the virus at the Merimbula sewage treatment plant three times in two weeks is of concern," Cr Fitzpatrick said.
"Since the virus fragments were first detected last week, the number of people getting tested for COVID-19 has been relatively low.
"It's critical we see higher COVID-19 testing numbers if we are going to find the source of these virus fragments."
In speaking with regional journalists at his daily briefing, Mr Barilaro said there had been "significant movement" in sewage detections across the state, including Merimbula as well as Cooma and a new detection reported Wednesday at Thredbo.
"The reality here is we know there's movement along that region, we know there are cases either in Sydney or in Canberra, we know there's that interconnectivity," Mr Barilaro said.
"It could be freight drivers that have been picked up elsewhere that have gone through the region. We don't know.
"This is the fishing exercise, or the needle in the haystack moment where you try to work through."
Mr Barilaro said sewage surveillance had been "a very important tool and one that has led to cases becoming known in a lot of scenarios".
However, "at this stage Merimbula still baffles us".
"It's always concerning when there's sewage detection. They are a good predictor of cases and have done so in the past.
"We can't rule out there isn't someone in the community with COVID, or maybe it is someone who has come through...possibly someone who has come through and then been picked up [in case numbers] elsewhere.
"So again, remain vigilant and get tested."
The Merimbula sewage treatment plant serves approximately 7000 people covering Merimbula, Pambula, Pambula Beach, South Pambula, Mirador, Berrambool and parts of Millingandi.
Cr Fitzpatrick said regularly emptied commercial pump-out tank systems in the shire also feed into the Merimbula plant.