Canberra: The Australian Capital Territory (ACT) plunged into a lockdown on Thursday after recording its first coronavirus case in the community in over a year.

Andrew Barr, chief minister of the ACT, announced that the territory would go into lockdown for seven days from 5 p.m. on Thursday after a man in his 20s tested positive for the virus, reports Xinhua news agency.

ACT Health said the man was infectious in the community with no known source of infection.

It is the first case of Covid-19 detected in the ACT community in more than 12 months.

"This lockdown decision is the result of a positive case in the territory, a case has been infectious in the community," Barr told reporters.

"We do not currently know the source of the infection, but extensive investigation has been under way for many hours.

"This is the most serious public health risk that we are faced in the territory this year, really, since the beginning of the pandemic," he said.

Under the lockdown, residents of Canberra and its surrounding suburbs will only be allowed to leave home for essential reasons including essential work, healthcare, vaccine appointments, grocery shopping and one hour of exercise per day.

General retail will be closed and hospitality businesses will move to take away only.

Masks will be made mandatory in public spaces.

Any ACT residents who visited an exposure site at the same time as the positive case have been ordered to isolate for 14 days.

"This will be the first time that the ACT has entered a lockdown of this nature since the early days of the pandemic," said Barr.

It makes Canberra another major Australian city currently subject to a lockdown with strict restrictions also in place in Sydney and Melbourne.

"We know from what we are seeing around Australia that the Delta variant of the Covid-19 virus is highly infectious, and life threatening," Barr said.

"For the sake of your health, your family's health and for the health of the community - it's critical that Canberrans take every precaution they can over the coming days."