The explosion of coronavirus cases in Sydney's northern beaches is likely linked to a single superspreader event, one expert has warned.
The cluster of Covid-19 infections has risen sharply to 17, prompting the NSW government to ask residents in the area to stay home and causing multiple states and territories to reimpose tough border restrictions.
A clinical epidemiologist from the University of Melbourne, Professor Nancy Baxter, said the outbreak was "extremely concerning" and warned cases will likely continue to rise.
"You have 17 people now being identified with this cluster. It looks like the super spreading event likely happened December 11," she told ABC's News Breakfast.
"You've now had seven days where this has been circulating. Although a number of people have been linked to the cluster, we don't know who the original source was.
"That person has been circulating for that period of time. I would anticipate a number of additional people will be identified as having Covid-19."
Baxter said the person likely had mild or no symptoms and may no longer be infectious but was probably unknowingly spreading the virus for a significant period of time.
NSW Premier Gladys Berejiklian revealed this morning that genomic sequencing indicated the virus was from an overseas source.
"Somehow that's obviously gotten out into the community," Berejiklian told Today.
"We are concerned that there could have been what we call seeding events, people from the northern beaches may not have known they had the virus, gone to other parts of Sydney and unintentionally affected people."
Northern beaches residents have been asked to stay home for the next three days as much as possible and avoid unnecessary travel outside the area.
They have also been asked not to visit friends or family in aged care facilities or hospitals unless essential, and to avoid high-risk venues including clubs, restaurants, churches and gyms.
There is speculation about whether the NSW government will follow the approach of the South Australian government and impose a harsh "circuit breaker" lockdown, similar to the restrictions imposed on Adelaide following the city's recent outbreak.
Baxter said it is possible the government will try and "ring-fence" the northern beaches to stop the cluster spreading.
"It's hard to know what's the right approach. They have recommended that even in the area stay at home for three days.
"They haven't made an order. I would say what will happen today, if there's more cases, it's going to change from a recommendation to an order. To try to ring-fence around this cluster, to try and get a handle on it. They don't have a handle on it yet."
She said a major rule that should be imposed is the wearing of masks, adding that everyone in Sydney should be wearing masks in public at this point.
Baxter said the recent easing of restrictions also increased the risk of these types of outbreaks occurring.
"So I think we all need to remember that we're living in a pandemic right now, and although we're doing much better in Australia than elsewhere, we can't let our guard down.
"It's not business as usual. We do have to maintain some restrictions and differences than our usual lives, even over the holiday period."
NSW's contact tracers are now racing against the clock to find the source of the virus and contain the cluster in time for Christmas.
Berejiklian said the next 24 to 48 hours were "absolutely critical" in getting a handle on the outbreak.
"That is why we are working night and day to get on top of it. But we need the public's help. The public has been fantastic up to this point," she told Today.
"We just need your help. If you live on the northern beaches, please follow the health advice.
"If you live in Greater Sydney, please take extra precautions. If all of us pull together and do the right thing, we have a greater chance of having a good Christmas and all of us, I think, deserve to be with our loved ones and have a break. But that will rely on how we pull together over the next few days."