A public health expert says Sydney's recent community COVID-19 case could be the "tip of the iceberg for a larger outbreak".
It comes after a man in his 50s tested positive for coronavirus after visiting a movie theatre, restaurants, a service station, and a meat store in Sydney's eastern suburbs while unknowingly infectious.
It's NSW's first community COVID-19 case in more than a month and health officials are now rushing to trace the source.
The infected person has not travelled overseas in recent times and does not work in any high-risk jobs such as in hotel quarantines or hospitals.
So far there has been no change to quarantine-free travel between New Zealand and New South Wales, but that could change.
Professor Michael Baker told Newshub the case is concerning because there is no clear link to the border.
"If this is a mystery case in the community and no sources are found... generally they are quite worrying because you have possibility of lines of transmission that you haven't detected, and that's where you do sometimes need to have a short sharp lockdown."
Baker said because the new case had a high viral load, it is unlikely to be historic.
"When you see an isolated case, one of the possibilities is that it's a historic case because the testing is so sensitive it can pick up viral debris sometimes months after the person has had the infection.
"Where you see a case like this and they're reported to have a high viral load that would suggest that they are a recent infection and they are actively infectious."
He said border workers who test positive are much easier to contact trace but isolated cases could signal a larger outbreak.
"If it's a border worker or someone who has recently entered the country... you've got a definite connection with the border... that means it's likely any outbreaks are small.
"In this instance, it doesn't seem to be the case so you have to say, 'Well how many other cases are out there that might have resulted in infection?' And this could be the tip of the iceberg for a larger outbreak - that's the worst-case scenario."
Baker said it's too early to tell whether quarantine-free travel will be suspended.
"It's very early days with this case, with the investigation, so what the authorities will be wanting to do is figure out how widespread this outbreak could be.
"In New Zealand, we now have this traffic light system - the three-level system for classifying outbreaks in other parts of the green zone...and then we do the risk assessment based on that system and act accordingly."
New Zealand's four-tiered lockdown alert system allows for different responses and restrictions depending on how many cases of COVID-19 each end of the travel door is experiencing.
When it's green, COVID-19 cases are at low risk of transmission - for example, any cases that do emerge are linked to the border. At this stage flights to and from Australia are "likely" to continue.
At orange, the chances of an outbreak is heightened - an unknown COVID-19 case has emerged and although it's "most likely" linked to the border, there's no way to be sure.
At this stage, quarantine-free travel from affected states would be paused for up to 72 hours.
At red - there are multiple cases of unknown source and the state would enter a longer period of lockdown. Here, flights would be suspended for an extended period.
Baker said it's possible this case will fall into the orange tier, but said there is not enough information to know yet.
"At this stage it would be hard to know where this outbreak would lie but it's certainly looking more in the middle range at the moment."
