A Gungahlin man in his 20s has been identified as the tested positive case. He was infectious in the community since Sunday.

The ACT government has started contact tracing, but the source of the virus is currently unknown.

The man visited a large number of venues while infected, including locations in Fyshwick, Braddon, Manuka, Mitchell, Cook, Kingston, Civic and Woden.

LIVE BLOG: Keep up to date with the latest developments as Covid returns to Canberra

During the lockdown, Canberrans have been asked to only leave their homes for essential purposes including essential employment, healthcare, COVID-19 vaccinations, shopping for groceries and supplies and up to one hour of outdoor exercise.

Everyone aged over 12 years must wear a mask at all times upon leaving home, including in workplaces.

General retail will be closed and hospitality venues will only be able to operate takeaway services. Businesses that have to remain open have been asked to actively prevent any browsing in their stores.

During the seven-day period, Canberrans should only be interacting with people from their household, ACT government has advised.

However, intimate partner visits are allowed and people who live alone can identify one other household that they can visit or receive visits from.

People are asked not to have any family gatherings or parties with friends.

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ACT Chief Minister Andrew Barr has urged residents not to leave their regions during what he said was by far the most serious public health risk the ACT has faced in the past 12 months.

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

"We have said throughout the outbreak in Greater Sydney that we would act quickly and decisively.

"We have seen that a short and immediate lockdown limits the potential spread of the virus, and is the best path to avoiding longer and more damaging lockdowns."

A person can leave their home for essential purposes with one other person or more than two people if all from the same household.

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A new testing clinic is being set up at the Brindabella Business Park and will be open from Thursday afternoon.

Capacity and operating hours will be significantly increased at the Weston Creek Walk-In Centre and EPIC drive through clinic.

Chief Minister Barr, Health Minister Rachel Stephen-Smith, Chief Police Officer Neil Gaughan, Chief Health Officer Kerryn Coleman and Education Minister Yvette Berry gave an update on the situation at 12.15pm.

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