The 47-year-old works mostly weekends at the Sofitel Wentworth in Sydney's CBD and the Mantra Sydney Central hotel in Haymarket.

Under the state's routine surveillance of quarantine staff, workers are tested after each shift.

The man tested positive late last night after testing as part of that scheme, and a second test confirmed the result early this morning.

The man had previously tested negative following his most recent shifts on March 5 and 6 and had reported having no symptoms.

NSW Chief Health Officer Kerry Chant said the "working hypothesis" was that he had been infected by a positive case in quarantine at the Sofitel between 7:00pm on March 6 and 7:00am on March 7.

"What we have done as a precaution is reached out again to the workers that were at that hotel at that time because, if this person was exposed, we are obviously thinking could anyone else have been exposed," Dr Chant said.

Urgent genomic sequencing is underway in an attempt to find links between the worker and a positive overseas case, with results expected late tonight or early tomorrow.

About 130 people who worked with the guard between 7:00pm on Friday March 12 to 7:00am on Saturday March 13 are self-isolating and getting tested.

"That allows us time to work through and ascertain the nature of interaction that this security guard would have had to those quarantine workers," Dr Chant said.

Contact tracing is also underway to identify anyone who has been in contact with the security guard since March 8.

His immediate household contacts are now in isolation and have been tested, with results so far coming back negative.

The security guard also works at another venue but Dr Chant said he mostly remained in an office in that role and work was underway to determine the level of risk involved.

NSW Health has issued alerts for the following venues, visited by the security guard, which it deems to be "low risk" at this stage:

* Angelo Anestis Aquatic Centre, Bexley, Saturday March 13

* Pancakes on the Rocks, Beverley Hills, on Saturday March 13

The man also caught a train from Hurstville to the city, arriving at 6:30pm on Friday March 12 and and city to Hurstville, leaving at 7:00am on Saturday March 13.

NSW Health Minister Brad Hazzard said the guard received his first Pfizer vaccine jab on March 2 and was due to receive the second dose this week.

"[This is] obviously good news, but you should be aware the antibody reaction required from vaccination doesn't get confirmed or doesn't achieve its maximum [until] some time after the first and second vaccination," he said.

"As we have said all along, vaccination helps but it does not necessarily stop you getting the virus."

The Pfizer vaccine is administered as two shots, spaced at least three weeks apart.

Dr Chant said the guard's immune system was unlikely to kick in for 12 to 14 days, or potentially longer, with the second dose designed to boost the immune response.

"While we are still learning how the vaccines impacted transmission, what we would hope for … is if you are vaccinated, you probably don't have such a high viral load, you are protected against severe illness," she said.

"Your viral load may well be lower and, therefore, it will potentially stop you transmitting it to the same degree. But a lot of this work is still being looked at and the science is being worked through as we literally speak."

Mr Hazzard said he was "Relatively relaxed" despite the positive case.

"He has been out and about, but not excessively," Mr Hazzard said.

"I am relatively relaxed today, knowing we have the systems in place that we have, and I am also relaxed about the fact that the testing did its job, picked up the gentleman."

NSW Police, which runs the hotel quarantine program, said it was working with NSW Health to ensure close contacts were tested, monitored and remained in isolation.

Story By | Bellinda Kontominas