NSW Premier Gladys Berejiklian has warned more locally acquired cases are likely after a Sydney man was diagnosed with the coronavirus with no idea where he caught it.

The unvaccinated man in his 50s from Sydney’s eastern suburbs had not been overseas and did not work in hotel quarantine, health or border control.

Nor had he been interstate.

Ms Berejiklian told NSW residents to “be on guard, no matter where you live” because the mystery infection had no known source.

“If there is one case and there’s no direct connection, we have to assume there’s other cases. Our response will be proportionate, as it always has been in NSW,” she said.

The results of genome sequencing are expected on Thursday as NSW Health contact tracers work to identify any contacts.

Also on Wednesday, fragments of the virus were detected in the sewage network at Marrickville, which has a catchment of about 42,000 people and takes sewage from Dulwich Hill, Marrickville, Summer Hill, Lewisham, Ashfield, Haberfield, Petersham, Lilyfield and Leichhardt.

“NSW Health is asking everyone in these areas to be especially vigilant in monitoring for symptoms, and if they appear get tested and isolate immediately until a negative result is received,” NSW Health said.

Professor Raina MacIntyre, an expert in global biosecurity and infectious disease from the University of NSW, told the ABC the mystery source of the infection was concerning, but it was likely to be traced back to hotel quarantine.

“It is worrying because it’s unknown where this man acquired this infection. Who did they get infected from? That’s the real question of interest,” she said.

The man attended a number of barbecue stores including Barbeques Galore at Annandale.

Infection mystery

NSW Health believe the Sydney man he had been infectious since last Friday.

“Everybody in the state needs to be on high alert,” Ms Berejiklian said.

“Anybody anywhere with the mildest of symptoms needs to come forward and get tested.”

The man has several close contacts who are being tested and isolating, NSW chief health officer Kerry Chant said.

He also visited several venues while potentially infectious, she said, but had been meticulous about checking in using QR codes.

The list of venues of concern includes barbecue shops in Silverwater, Annandale and Casula, a meat store in Bondi Junction and a petrol station in Mascot.

The man also attended a screening of The Courier at Event Cinemas Bondi Junction on Friday before attending Figo Restaurant at Rushcutters Bay.

• See the updated NSW Health list here

People who have attended those venues at various times between April 30 and May 4 should get tested and self-isolate until they receive further advice.

“This person did everything right, but it goes to show that we can’t take a single thing for granted,” Ms Berejiklian said.

The man has not yet received a COVID-19 vaccine.

Ms Berejiklian also said it was too early to say if NSW’s coronavirus-related restrictions need to be tightened.

States react

Queensland’s chief health officer Jeannette Young said anyone in Queensland who had visited any of the listed Sydney venues in recent days should quarantine immediately and get a COVID-19 test.

Western Australia’s chief health officer Andrew Robertson mirrored that, saying anyone in WA who had visited any of the listed Sydney venues should get tested for COVID-19 and quarantine for 14 days.

“We believe any risk to WA remains very low, but the situation highlights the importance of remaining vigilant to prevent the chance of any spread of the virus or community transmission in this state,” Dr Robertson said.

-with AAP