The Hunter has recorded its first new coronavirus case in 76 days after a returned traveller tested positive in Newcastle following his release from hotel quarantine.
Health authorities said the man had returned a negative test result on day 10 of his quarantine but had developed COVID-19 symptoms after returning to Newcastle on Sunday.
They said it was unusual for someone to test positive after the mandatory two-week quarantine period.
"More than 50 per cent of people develop symptoms by day five and 98 per cent by day 12, so it's very rare that someone develops symptoms this late," Hunter New England Health public health physician Dr David Durrheim said in a video posted on social media.
The recent outbreak that has closed the Victorian border and forced Melbourne back into lockdown is linked in part to a breakdown in security protocols at quarantine hotels.
Asked about the specifics of the Newcastle case, a NSW Health spokesperson said NSW Police had released the man on his 14th day of quarantine.
"He developed symptoms on his 15th day and was promptly tested and he and his contacts isolated," the spokesperson said.
HNEH said it would not release details of the man's movements because it was satisfied he had not mixed with large numbers of people.
NSW recorded seven new coronavirus cases in the 24 hours to 8pm on Monday. The other six were travellers who tested positive entering hotel quarantine in Sydney.
The Hunter last recorded a new case on April 22, when Dr Durrheim said the virus would have to be imported to the area to flare again.
By early June, the region had no active cases.
Dr Durrheim said on Tuesday that the man who had tested positive should be congratulated for seeking a test after developing "very mild" cold-like symptoms.
"This is a really good reminder to all of us that at this stage, when we have had an influx of people from Melbourne and possibly from the hot spots there, that if we develop any symptoms of colds or flu that we should immediately make sure we don't spread it in the community and get ourselves tested," he said.
The confirmation of a Newcastle case came hours before the Victorian government announced a lockdown on metropolitan Melbourne and Mitchell Shire, immediately north of the capital, after the state reported a daily record of 191 new cases.
About 800 NSW Police and Australian Defence Force personnel are due to enforce a state border closure from midnight on Tuesday.
Premier Daniel Andrews announced Melbourne would re-enter stage-three restrictions for six weeks from midnight on Wednesday.
Residents must stay at home unless they need to work, study, shop, exercise or give or receive care.
Hospitality businesses will return to takeaway only, people will not be allowed visitors at their homes and public gatherings will be limited to two.
Mr Andrews said Victoria was in a more serious predicament than it was several months ago at what appeared then to be the pandemic's peak.
"I know a lot of people aren't scared because this feels like something happening to other people in other parts of the world," he said at a media conference.
"But you should be scared of this. I'm scared of this. We all should be."
He extended the school holidays by a week for all but year 11 and 12 students and said the government would announce this week how the rest of the school term would operate.
University of Newcastle infection prevention specialist Professor Brett Mitchell told the Newcastle Herald that renewed outbreaks in Australia were "inevitable".
"We haven't eradicated COVID from Australia," he said.
"Obviously the one in Victoria is a concern because of the large numbers of people in different population groups and postcodes.
"I'd be surprised if we didn't see these type of things occurring. Part of that is we've relaxed restrictions across the country and, in doing that, we knew that these types of things would occur."
Mr Andrews warned Victoria's ability to trace patients' contacts would be limited if case numbers continued to grow.
Professor Mitchell agreed that a high testing and tracing capability did not guarantee that large outbreaks like those seen overseas would not happen.
"There comes a time when contact tracing becomes too difficult to do, when you're talking about sheer numbers of people who cannot possibly be followed up."
In an article he co-authored for The Conversation, a news website, Professor Mitchell wrote that "moving in and out of various levels of restriction may just be part of life as we know it in 2020, and likely 2021".
"It's hard, but this is just how vigilant we have to be until a vaccine is found."
He said government authorities would need to "think carefully about how to manage frustration".
While you're with us, did you know the Newcastle Herald offers breaking news alerts, daily email newsletters and more? Keep up to date with all the local news - sign up here