A makeshift COVID-19 testing clinic set up outside a Sydney pub is working to determine the extent of an emerging cluster, with five cases now confirmed.
NSW Health said on Saturday a man who visited The Crossroads Hotel in Casula on the evening of July 3 had passed on the virus to three people in his Blue Mountains household.
Those close contacts, who are self-isolating, will be included in Sunday's NSW case numbers.
The man's visit to the pub is the only known link between him and a southwestern Sydney woman who tested positive earlier this week.
Anyone who attended the hotel last Friday is being asked to self-isolate and get tested immediately if they develop even the mildest symptoms.
The pub is closed for deep-cleaning while a huge queue of people lined up outside the clinic set up in the car park.
Major hospitals in the area in Liverpool, Campbelltown and Fairfield have also extended the opening hours of their testing clinics.
"We are at a critical point in the fight to contain the virus," NSW Health director of health protection Jeremy McAnulty said on Saturday.
"It's essential that the community works together to limit the spread of the virus, by always maintaining good hand hygiene, adhering to physical distancing rules whenever possible and getting tested whenever symptoms develop, however mild.
"And if you've been in Victoria in the last couple of weeks, don't mix with other people until two weeks have passed."
The male pub patron and a man in his 20s from Melbourne, who had towed his caravan up from Victoria on July 7, were among seven new cases recorded in NSW to 8pm on Friday.
The other five were returned travellers in hotel quarantine.
Dr McAnulty said the Victorian traveller's three close contacts - his partner, who has tested negative, and two friends - are in isolation.
Australian Medical Association NSW vice-president Andrew Zuschmann backed the calls for caution.
"Because of what's happened there (in Victoria) and because of the porous nature of the border prior to it being closed, people in NSW are going to need to be very careful over the next few weeks," Dr Zuschmann said.
"It's likely we won't know for sure whether community spread is going to be a problem here for a while.
"You need to act like you're already infected and treat people not from your own household as though they are infected."