The state's 12-day streak without locally acquired coronavirus cases ended on Wednesday after a Sydney airport driver was confirmed to have the virus and two mystery cases popped up on the northern beaches.
Another two cases on the northern beaches have since been identified, Premier Gladys Berejiklian confirmed on Thursday, taking the area's COVID-19 cluster to four people.
Seven venues on the northern beaches peninsula have been identified as places recently visited by an infected man and woman. People started queuing up at Mona Vale Hospital on Thursday to get tested for the virus.
"We have seen an increase in testing overnight ... we want to get on top of this and don't want this concerning us in the last few days before Christmas," Ms Berejiklian told reporters.
The latest outbreak has prompted concerns about whether the recently-opened Queensland and WA borders will again be shut to people from NSW over Christmas.
WA Premier Mark McGowan didn't rule out re-imposing restrictions if further infections were detected in NSW, saying "if the advice comes back that we need to put up a hard border, then we will".
Acting Queensland Premier Steven Miles said the government was not considering fresh border restrictions at this stage, saying the next 48 hours would be important.
The first two northern beaches cases - a woman in her 60s and a man in his 70s - are close contacts of each other but authorities have not found a source for their infections.
Alerts are out for several areas in the northern beaches, particularly Avalon and Palm Beach, after the two moved through the community while contagious.
NSW Health is urgently undertaking genomic sequencing and contact tracing to stem the damage and identify the source of the infections. The genome sequencing results will be available on Thursday or Friday.
The cases were identified just hours after a 45-year-old driver transporting international air crew members to and from Sydney Airport and their hotels was confirmed COVID-positive.
NSW is considering changes to quarantining requirements for international air crew, the state's Health Minister Brad Hazzard says.
NSW Chief Health Officer Kerry Chant told reporters on Wednesday the southwest Sydney man worked only with air crew members and was not involved in regular taxi services for the public. He also wore a mask while working.
But Mr Hazzard said a stronger regime for international air crew members may be required to eliminate future risk - likely by placing air crew in full hotel quarantine until their flight back out of Australia, which usually occurs within 72 hours.