Six new cases were acquired overseas to 8pm Sunday night, with a total of 13,768 tests reported in that time period compared with the previous day's total of 18,024.

NSW Chief Health Officer Dr Kerry Chant said it's still a mystery as to how a Sydney man in his 50s contracted an Indian variant of COVID-19 which prompted a return of certain restrictions.

Dr Chant, who was speaking at a new mass vaccination centre in Sydney Olympic Park on Monday, said authorities were still working to find the "missing link" but praised the patient for being so cooperative.

IN OTHER NEWS:

"He's really gone above and beyond to help us," Dr Chant said. "We have reviewed CCTV footage ... he was in the city area and some of his walking routes did take him down around the precincts of the quarantine hotel ... but our lines of inquiry are continuing."

What they do know is the man and his partner are the first known locally acquired cases of the Indian B.1.617 variant of the virus in NSW, with genomic sequencing revealing the infection was linked to a returned overseas traveller from the US who was in quarantine at the Parkroyal hotel at Darling Harbour.

Dr Chant said whereever the encounter occurred it would have been "fleeting" so there is concern others in the community may be carrying and spreading the virus unknowingly.

"What were concerned about is there might be broader transmission in the community and for it not to have been detected - we've seen that, for instances in cases in Avalon [late last year]," she said.

Dr Chant defended the decision announced on Sunday to continue restrictions for Wollongong and Greater Sydney (including the Central Coast and Blue Mountains) unftil 12.01 am Monday May 17, despite no other new community transmitted cases.

She said they wanted to focus on the most "minimal impact with the greatest outcome" so people would not be greatly impacted in their daily lives, though have allowed patrons to retail and hospitality businesses to be free of a mask though staff must wear one.

"The rationale for hospitality workers to be wearing it and not patrons is because clearly when you're going to those hospitality venues you're eating and drinking and it makes no sense," Dr Chant said.

"But what we've seen in a number of our other scenarious is the hospitality workers can often get infected from their patrons but then because they encounter so many people in a day they then can spread it. In retail, we've seen actually not many transmissions in retail settings - the exception is when the actual worker is actually positive."

Restrictions include limits on visitors to households and aged care centres, restrictions on singing and dancing, bans on standing while drinking plus mask wearing on public transport and at indoor venues (except retail and hospitality customers). Patrons will need to mask up in the gaming area of venues.

A full list of venues of concern connected with the two recent locally acquired cases is available on the NSW Government website.

There are more than 300 COVID-19 testing locations across NSW. To find your nearest clinic, visit: https://www.nsw.gov.au/covid-19/how-to-protect-yourself-and-others/clinics or contact your GP.