The Hunter New England region yesterday recorded its first new coronavirus case in 76 days after a returned traveller tested positive in Newcastle following his release from hotel quarantine.
But a health academic said renewed outbreaks are "inevitable", while authorities praised the man for getting tested very rapidly after he came down with symptoms under conditions they said were "very rare".
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.
The border was slammed shut last night, and is now patrolled by about police and personnel from the Australian Defence Force.
University of Newcastle infection prevention specialist Professor Brett Mitchell told ACM 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".