Australia's most populous state on Monday reported its lowest one-day rise in new Covid-19 cases in three days, stoking cautious optimism that authorities have contained an outbreak in Sydney's northern beachside suburbs.
New South Wales (NSW) said 15 people had tested positive for Covid-19 in the past 24 hours, down from the 36 infections detected a day earlier and bringing the total cases in the northern beaches outbreak to 83.
It confirmed it had detected cases of the fast-spreading new coronavirus strain that has forced Britain to reverse plans to ease curbs over Christmas.
NSW Premier Gladys Berejiklian said it was too early to say a larger outbreak had been averted as northeastern suburbs entered the third day of a five-day lockdown.
"I'm pleased with what we've seen overnight, but again, it's volatile," Berejiklian told reporters in Sydney.
The government would provide an update by Wednesday on "what Christmas and the next few days look like" in terms of further containment measures beyond those already imposed on the northeastern suburbs, she added.
However, other states and territories have moved quickly to close borders to all of Sydney's 5 million residents, throwing Christmas travel plans into disarray.
"2020 is not done with us yet," Australian Prime Minister Scott Morrison told reporters in Canberra.
"The events of the past few days, I have no doubt, are incredibly frustrating and disappointing for people all around the country who had plans in place to get together and move in between states."
NSW imposed a lockdown on Friday in Sydney's northern beaches area, home to more than 250,000 people, after the cluster of cases emerged, centred on two dining and entertainment venues in the seaside suburb of Avalon.
More than 80 locations including cafes, gyms, casinos and supermarkets across Sydney were identified as having been visited by people with confirmed cases.
Authorities have urged anyone who attended the venues to immediately get tested and self-isolate, while dozens of domestic flights due to leave Sydney were cancelled on Monday.
With travel routes upended, Australian states and territories warned their own residents to leave NSW and return quickly if they wanted to avoid mandatory quarantine.
Concrete barriers will once more be erected at the border between NSW and Queensland state, with Australian police and military personnel deployed at many state borders to enforce the new rules, the Queensland state government said.
Lawmakers warned people to expect long delays across state lines.
Desperate to contain the spread, NSW has opened dozens of new testing sites, some of which are running 24-hours a day.
NSW health authorities said more than 38,000 tests had been conducted in the past 24 hours, a new record.
The source of the infection is unknown although Berejiklian said tests showed the strain of Covid-19 found in Sydney's northeast was almost identical to that found in an unnamed woman who arrived in Sydney on Dec. 1 from the United States.
All arrivals enter mandatory hotel quarantine on their arrival, which Australia credits as central in it avoiding high coronavirus numbers compared with other developed nations.
While in hotel quarantine people are tested for Covid-19.
Australia has recorded just under 28,200 cases and 908 deaths since the pandemic began.
Sydney's outbreak has forced the abandonment of the annual Sydney-to-Hobart yacht race for the first time in 76 years, and other cities are on standby to host the cricket test between Australia and India scheduled to start at the Sydney Cricket Ground on Jan. 7.