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Australia's capital Canberra was ordered into a seven-day lockdown Thursday, after a single COVID-19 case was detected in the city that has largely avoided virus restrictions.

About 400,000 people in the nation's political hub will be under stay-at-home orders from 5:00 pm local time, joining millions more already under lockdown in Australia's southeast.

"This is the most serious public health risk that we are faced in the territory this year. Really, since the beginning of the pandemic," Australian Capital Territory chief minister Andrew Barr said.

He added that the COVID-positive person had been in the community while infectious.

Authorities also appealed to residents to avoid panic buying, as stores began to fill with maskless shoppers shortly after the announcement.

"There is no need for panic buying. And our key message to people is please be patient, be kind with one another, and be thoughtful," said the capital's health minister Rachel Stephens-Smith.

Canberra has not been in lockdown since a nationwide shutdown in the early stages of the pandemic in 2020.

After months of pursuing a "COVID zero" strategy, Australia is struggling to contain multiple outbreaks of the highly transmissible Delta variant.

More than 10 million people in the country's biggest cities, Melbourne and Sydney, are currently in lockdown as authorities try to bring case numbers down.

Much of western New South Wales state was also placed under late Wednesday, amid concerns for a sizeable Indigenous population feared more vulnerable to coronavirus.

"I ask all our Aboriginal community as well to please stay at home, come forward for a test if you have symptoms and of course please get vaccinated with any available vaccine as soon as you can," New South Wales Health's Marianne Gale said.

In Sydney, the epicentre of the outbreak, almost 6,500 cases and 36 deaths have been recorded since a cluster emerged in mid-June.

The is expected to spend at least nine weeks under stay-at-home orders, with several hotspot suburbs placed under harsher restrictions Thursday.

Australia won global praise for its successful coronavirus response in the early stages of the pandemic, and most of the country was enjoying few restrictions by late 2020.

But a glacial vaccination rollout has been no match for the Delta variant, leaving cities and towns reliant on repeated lockdowns as they attempt to stamp out the virus.

The nation has recorded more than 37,500 cases of COVID-19 and 946 related deaths to date in a population of 25 million.