The Hornsby Hospital staffer worked on August 6 from 11am to midnight while infectious, the Northern Sydney Local Health District said in a statement on Saturday night, but was wearing a mask at all times while in contact with patients.
The worker was asymptomatic at the time but became unwell after their shift.
Staff in contact with the worker have been identified and told to self-isolate for 14 days, while those who attended the hospital on August 6 are being chased up.
The worker was one of 10 new COVID-19 cases recorded in the 24 hours to 8pm on Saturday from 31,681 tests, with just one case in hotel quarantine.
Two cases remain under investigation, NSW Health said in a statement on Sunday.
NSW residents are now largely confined to their own state after Queensland closed its border, following similar moves by Western Australia, South Australia and Tasmania.
NSW has ramped up its own travel restrictions and is forcing residents returning from coronavirus-hit Victoria into two weeks of hotel quarantine.
Entry to NSW from Victoria is now restricted to flights landing at Sydney Airport, except for border community residents with permits.
Meanwhile, a second student at Tangara School for Girls in Cherrybrook has come down with COVID-19, with all students from years seven to 12 sent into two weeks of self-isolation. The secondary campus will remain closed until August 21.
Motorists are seen approaching a checkpoint at Coolangatta on the Queensland- New South Wales border. Source: AAP
Another confirmed virus case on Sunday is a household contact of a Tangara student.
Our Lady of Mercy College in Parramatta on Saturday night also confirmed a schoolgirl had contracted COVID-19, with deep cleaning and contact tracing now underway.
NSW Health has issued advice to shoppers who attended Bunnings in Campbelltown in Sydney's west on August 4, 5 and 6 to be alert to COVID-19 symptoms after an employee tested positive. The worker wore a mask during their shifts.
NSW recorded nine new COVID-19 cases in the 24 hours to 8pm on Friday - the lowest number in two weeks - but Premier Gladys Berejiklian on Sunday warned against complacency.
"No matter how tough, well-organised and well-resourced we are, we rely on all citizens doing the right thing, all of the time," she wrote in an open letter published in The Sunday Telegraph.
"It is sadly no exaggeration to say that when it comes to COVID, complacency could literally be the difference between life and death. Just one person can put the entire state in jeopardy."
There are currently 111 COVID-19 cases connected to the Thai Rock Wetherill Park cluster, 59 to the funeral events cluster and 34 linked to the Potts Point cluster.
Eight patients are currently in intensive care in NSW, with six on ventilators.
"While most cases in the past week have been associated with local clusters and close contacts with known cases, 11 cases have not been linked to known cases, indicating COVID-19 is circulating in the community," NSW Health's Dr Jeremy McAnulty said in a statement.
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