The outage left some NSW residents and businesses confused about how they should check in, after the government made using the Service NSW app mandatory from January 1.

A spokesperson for the Department of Customer Service told AAP the app experienced an unexpected outage for about two hours on Thursday afternoon, but the problem was now resolved.

"It is vital that customer contact details are still collected digitally. In the event of an outage, customers and businesses are encouraged to use the Service NSW check-in webform for all check-ins," the spokesperson said in a statement.

Scanning a QR code should open the webform, which was not impacted by the outage.

The department suggested using a spreadsheet or other form of digital entry to track check-ins in the event of another outage. Any paper records must be entered into an electronic format within 12 hours.

"Service NSW apologises for any inconvenience caused by the outage and thanks customers for their understanding during this time," the spokesperson said.

The outage happened after NSW again recorded zero new locally acquired cases in a 24-hour period, from almost 28,000 tests.

NSW last recorded zero new local COVID-19 cases on Monday, which was the first zero-case 24-hour tally in almost three weeks.

But one COVID-19 case was uncovered on the northern beaches after the 8pm cut-off time - a man who has already served a period in quarantine and whose infection is not yet directly linked to the area's Avalon cluster.

He lives in the northern zone of the northern beaches, which remains under lockdown.

Chief Health Officer Dr Kerry Chant said the man did not present symptoms until after he completed his quarantine period, at which point he sought re-testing. He tested COVID-negative at the end of his quarantine.

She said the link between the man and the cluster remains unknown, but acknowledged there were few new opportunities for him to be infected.

"It does highlight that with that level of community transmission in that northern zone ... that is the risk we want to make sure we tackle, very promptly," Dr Chant told reporters on Thursday.

Earlier this week, an 18-year-old COVID-positive man's regional NSW camping trip through the state's west put several towns on alert.

The man visited the western NSW towns of Orange, Nyngan and Broken Hill on his trip, with several venues in those towns subject to alerts.

The man had attended the Berala BWS bottle shop on Christmas Eve, now the centre of the 16-strong Berala cluster in Sydney's west.

Authorities on Wednesday expanded the suburbs where residents and workers are locked out of attending Sydney's Australia-India cricket Test from Thursday, adding Wentworthville and Belmore.

Those who go to the SCG need to wear a mask at all times except when eating and drinking, Health Minister Brad Hazzard said on Thursday.

He also said that attendees of other sport events in the future, such as the football A-League, would also be obliged to wear a mask.

Health authorities on Wednesday evening issued COVID-19 casual-contact alerts for the Nostalgia Factor Shop in Kangaroo Valley and Taj Indian Restaurant in Huskisson on December 27, to Sydney's south.

Participants in a cricket match at Ollie Webb Cricket Ground in Parramatta on December 28, meanwhile, have been told to get tested and isolate for 14 days regardless of the result - beefing up prior orders.

Acting Premier John Barilaro on Thursday confirmed he would not stand in for holidaying Premier Gladys Berejiklian when National Cabinet convenes on Friday to discuss the contagious UK COVID-19 variant.

Meanwhile, police in a statement on Thursday said they have issued 12 fines to those refusing to wear a mask, including one man in Blacktown twice.

Australian Associated Press