Anyone who has been to Perisher in the past two weeks has been advised to look out for symptoms of COVID-19 after traces of the virus were found in sewage during tests by NSW Health.
It comes as there were 18 new cases of COVID-19 diagnosed in NSW up to 8pm last night, of which six are still under investigation.
Of the 18 cases, two are linked to funeral clusters, four are associated with the Thai Rock restaurant cluster at Wetherill Park, which has risen to 89; four are associated with The Apollo restaurant at Potts Point and two are returned travellers in hotel quarantine.
NSW Health said that due to the "growing number of cases" it is asking people who live in, or have visited, a number of areas in the past two weeks to get tested if they have even the mildest of symptoms, such as a runny nose or scratchy throat.
These include Potts Point, Carnes Hill shops, Prestons, Bonnyrigg, Wetherill Park, Mt Pritchard, Bankstown City Plaza, Cabramatta and Perisher.
NSW Health's Dr Jeremy McAnulty said while there had been no cases reported in Perisher or Thredbo, a research program to test sewage for traces of COVID-19 had turned up a positive result.
Dr McAnulty said initial samples collected from Perisher on July 22 was positive for COVID-19.
He said a positive sample could be the result of someone "shedding the virus", which could occur for up to four weeks.
"It's important that anyone who has been to the area get tested even if they have the mildest symptoms," he said.
Dr McAnulty said a COVID-positive person visitedFrank's Pizza Bar Restaurant in Camperdown on July 26 from 6pm to 8pm.
Anyone who was at the restaurant at that time should monitor for symptoms.
Three cases have been linked to the Mounties club at Mount Pritchard.
Anyone who attended the club from 6pm Wednesday, July 22, to 3am on Saturday, July 25, must self-isolate and get tested, regardless of symptoms.
Fort Street High School at Petersham is closed today due to a possible case of COVID-19, as is Everlearn Childcare Centre at Prestons.
A student of Mary Immaculate Catholic Primary School at Bossley Park and a student at Freeman Catholic College at Bonnyrigg have both tested positive for COVID-19.
NSW Health said both students had been in isolation since the weekend and neither had attended school since deep cleaning was carried out. Additional contact tracing is under way.
A prisoner at Parklea Correctional Centre has also tested positive.
The total number of cases in NSW stands at 3547.
NSW Health is treating 100 cases. Five people are in intensive care, of which one is being ventilated.