A person who visited Horderns Restaurant at Milton Park Country House Hotel and Spa in Bowral has tested positive for COVID-19.
Their movements were confirmed by NSW Health on Sunday.
Anyone who visited the restaurant on Sunday, August 2 between 7.45pm and 9.15pm is considered to be a casual contact.
Red more: More NSW COVID cases from unknown source
People should immediately get tested and isolate if symptoms occur, however mild.
NSW Health said the Bowral case was linked to the Tangara School for Girls cluster.
Guest services manager David Fast said the venue had strict COVID-19 procedures in place.
Precautions include a questionnaire on arrival, temperature checks, regular deep cleans as well as the use of antiseptic wipes and gloves.
Staff also wear masks during dinner and bar services.
"This was an isolated incident. No other staff or guests have tested positive," Mr Fast said.
"It was just the restaurant that was the main concern."
Five new cases of COVID-19 were diagnosed in the 24 hours to 8pm last night, bringing the total number of cases in NSW to 3761.
NSW Health has reported the death of a man in his 80s who had COVID-19.
There have now been 54 deaths in NSW from confirmed COVID-19.
There are 3761 confirmed cases including interstate residents in NSW health care facilities. A total of 1,832,980 tests have been carried out.
There were 24,093 tests reported in the 24-hour reporting period, compared with 27,389 in the previous 24 hours. v
Australia hits grim record with 25 deaths
Australia has suffered another grim coronavirus milestone, with Victoria announcing a record 25 deaths in one day.
But new Victorian case numbers were again under 300 as the state's outbreak continues to show tentative signs of easing.
The country's previous worst day for deaths was 21 last Wednesday, all in Victoria, and these days are the only two days when the number of fatalities has been above 20.
Monday's fatalities take the state death toll to 334 and the national figure to 421.
It was a significant spike after only four deaths on Saturday and 16 on Sunday.
Authorities have warned of ongoing deaths in Victoria even as the new case numbers decline amid a stage-four lockdown for metropolitan Melbourne and a stage-three lockdown in regional areas.
Monday's new cases continued the downward trend in case, with 303 and 279 over the weekend and no day above 400 since last Wednesday.
It comes as all residents at an inner-suburban Melbourne special accommodation facility were being moved to hospitals because of a virus outbreak.
The Hambleton House facility in Albert Park has had security staff stationed outside because of concerns residents were walking nearby streets without face masks.
Health Minister Jenny Mikakos says residents still at the facility will be moved to hospitals by Monday.
Hambleton House is a 34-bed facility for residents with mental health problems or behavioural issues.
While aged care facilities and abattoirs have been the focus of Victoria's outbreak, as of Sunday there were also 81 active cases residential disability accommodation.
Meanwhile, Victorian officials are working to better understand COVID-19 infection rates among the state's health workforce as more public sector nurses are deployed in aged care homes to cope with rising cases.
Premier Daniel Andrews says details should be released early this week to answer some fundamental questions in relation to health staff.
On the latest figures, 1164 health workers are infected with the virus.
"We know how health workers are positive but where did they get it from," Mr Andrews said.
"Did they get it from work, did they get it in the community?"
The analysis should also provide some breakdown on which workers were most at risk and why.
Australian Associated Press