Canberra recorded four COVID-19 cases by Thursday evening and was bracing for more in coming days, after the city entered a seven-day lockdown at 5pm on Thursday afternoon.

ACT Chief Minister Andrew Barr made the lockdown announcement after a Gungahlin man in his 20s tested positive to the virus and wastewater sampling found traces of COVID-19 in the ACT. There was no known source of the infection.

Three more people tested positive for the virus on Thursday, from nine close contacts of the original case.

"We know from what we are seeing around Australia that the Delta variant of the COVID-19 virus is highly infectious, and life threatening. For the sake of your health, your families health and for the health of the community - it's critical that Canberrans take every precaution they can over the coming days," Mr Barr said.

Canberrans will only be allowed to 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. Non-essential retail will be shut during the lockdown.

Supermarket shelves were stripped bare in some parts of Canberra on Thursday afternoon and wait times for some Covid testing centres blew out to eight hours.

Parents have been urged to keep their children home from school and childcare if they can, while a regional border bubble will mean residents in nearby NSW towns will be given a standing exemption to enter the territory for essential reasons.

Canberrans who lose work will be eligible for Covid disaster payments, while support grants will be made available to businesses whose turnover declines by 30 per cent or more. The support grants will be jointly funded by the ACT and the Commonwealth.

Fourteen exposure sites were identified, including several shops at the Canberra Outlet Centre in Fyshwick, cafes, and a Fyshwick gym.

Fiction Bar in the city centre, along with the Woden Church of Pentecost were also identified as exposure sites, which experts said were high-risk settings.

Photos posted to social media show Fiction Club, located underground in the Cinema Center on Bunda Street, hosted a crowd of people on Saturday night and into the early hours of Sunday morning, when a confirmed positive case of coronavirus visited the venue.

Chief Health Officer Dr Kerryn Coleman would not be drawn on whether the man had recently been to Sydney, saying investigations were continuing.

Professor Mary-Louise McLaws, a UNSW epidemiologist who advises the World Health Organization, said exposure sites at nightclubs and churches were "unequivocally" among the worst possible exposure site locations.

"We know that this virus can spread in really unusual manners, regardless of whether it's Delta. It depends on the airflow. You don't need to be close to a source, a person infected, when you're indoors like that, with potentially air conditioning airflow," Professor McLaws said.

Professor McLaws said given Fiction was an underground nightclub, it was "very unlikely" its ventilation system provided enough air flow change to prevent the spread of coronavirus.

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Fiction Club was contacted for comment but did not respond to emails or messages.

Associate Professor Sanjaya Senanayake, an infectious diseases specialist at the Australian National University, said he expected Canberra would record more cases from the exposure sites but he was hopeful the city could avoid a protracted lockdown.

"If there were opportunities where there were enclosed spaces with lots of people, particularly if those people were shouting, singing, breathing heavily - so, like a nightclub, or potentially a church if people a signing hymns - those sorts of activities could lead to a number of cases," Dr Senanayake said.

Dr Senanayake said strong public compliance with health orders would help authorities to move quickly to contain the spread. "I'd be hopeful that we'd have a better outcome and a shorter outbreak than the other places," he said.

Dr Senanayake said the exposure sites showed it was important for younger people to be vaccinated, as they were often more likely to attend potential superspreader events.

"I think many of us have been saying for a long time have been saying the most vulnerable, the ones who are susceptible, to severe Covid are vaccinated early on, but the next group should be that young and socially mobile group, because they're the ones who account for the bulk of the cases, and they're the ones who spread most of the cases," he said.

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