This follows news that BP Mittagong was closed for a deep-cleaning early Wednesday morning after a positive COVID case walked into the store on Tuesday night.

On Tuesday, there was one confirmed case of COVID-19 reported in the 2577 postcode on the NSW Health website.

It is the second time this month that such fragments have been detected, with the previous finding coming on August 5.

Mittagong was among four regional towns singled out after sewage testing.

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"NSW Health's ongoing sewage surveillance program has detected fragments of the virus that causes COVID-19 at the Forster, Bateau Bay, Yamba and Mittagong sewage treatment plants in recent days," said the announcement.

"The Forster sewage treatment plant serves around 12,000 people, the Bateau Bay sewage treatment plant serves around 39,000 people, the Yamba sewage treatment plant serves about 6,500 people and the Mittagong sewage treatment plant serves about 14,000 people.

"These detections are of particular concern and everyone in these areas is urged to monitor for the onset of symptoms, and if they appear, to immediately be tested and isolate until a negative result is received."

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While Forster, Yamba and Bataeu Bay were mentioned in the 11am press conference this morning, the Mittagong treatment plant was only referred to in a NSW Health Facebook post.

This morning's press conference also revealed that 681 cases of COVID-19 had emerged until 8pm last night, following 119,000 tests, and regional NSW's lockdown has been extended another seven days until August 28.

If you are directed to get tested for COVID 19 or self-isolate at any time, you must follow the rules whether or not the venue or exposure setting is listed on the NSW Health website.

It remains vital that anyone who has any symptoms or is a close or casual contact of a person with COVID-19, isolates and is tested immediately. When testing clinics are busy, please ensure you stay in line, identify yourself to staff and tell them that you have symptoms or are a contact of a case.

Please check the NSW Government website regularly, and follow the relevant health advice if you have attended a venue of concern or travelled on a public transport route at the same time as a confirmed case of COVID-19. This list is being updated regularly as case investigations proceed.

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