Health authorities are still working to understand the origin and extent of the outbreak, with more than 5700 close contacts identified across a rapidly growing list of exposure sites.

There were nine active cases in the Canberra community to noon on Sunday.

Chief health officer Dr Kerryn Coleman said contact tracers were working through rich information about the outbreak.

"I've been involved in several public health investigations, and I think we're well on track in terms of what we would expect to be having a look at and what we're seeing at the moment," Dr Coleman said.

But Dr Coleman began laying the groundwork for an extension to the lockdown, warning Canberrans the virus had a 14-day incubation period.

"I'm making no promises at the moment. I do think people need to be realistic," she said.

Fiction nightclub, an underground Bunda Street venue epidemiologists have described as high risk for transmission, has become a likely transmission site.

The time for possible exposure was expanded to 9pm on Saturday, August 7, to 4.45am on Sunday, August 8, meaning potential close contacts have been in the community for up to a week.

Dr Coleman did not explain why the time was expanded, but said it was reassuring the lockdown had limited activity in the community.

"I won't comment on the specific exposure site location, but this is what happens in public health. When we get different sources of information - we uncover, people remember - sometimes those exposure locations expand out, sometimes they contract down," she said.

The second case announced on Saturday, a man in his 20s, was linked to the club; the person was at the venue between 1am and 4am on Sunday.

Dr Coleman said it was essential anyone at the nightclub during the exposure period get tested and remain isolated.

Meanwhile, another man in his 20s tested positive, with no known links to other cases.

The source of infection in a 14-year-old Gold Creek School student also remains a mystery. A close contact of that student also tested positive.

"There are investigations ongoing for one or two others about whether they're possible links," Dr Coleman said.

The first identified case in Canberra, which prompted the lockdown, was on Saturday linked to several in the Greater Sydney outbreak through genomic sequencing.

Chief Minister Andrew Barr said the ACT's 14,000 or so extra doses of the Pfizer vaccine, secured by the Commonwealth from Poland, was helpful but not a game changer, equivalent to an extra week's supply for the ACT.

Mr Barr said it would have little effect if the extra doses were prioritised for young people, currently ineligible for Pfizer vaccinations.

"It's just not enough to make any inroads, any significant inroads, in the cohort that's approaching 80,000 Canberrans. So to double-dose, you'd need 160,000 doses," he said.

Mr Barr urged Canberrans to keep doing what they were doing, by staying at home and following the public health orders.

"It's been so pleasing to see the seriousness in which Canberrans are responding to the challenge that we have. I understand people are concerned and anxious, it's a challenging time for everyone," he said.

"The response from the Canberra community has been fantastic."

Mr Barr welcomed the lockdown of regional NSW areas, and said the future of the ACT's border arrangements with its surrounding state would depend on NSW's decision on whether the state treated Greater Sydney differently to regional areas.

"From what I've heard from their press conference earlier, they're undertaking Operation Stay At Home. Excellent idea. It would have been even better about seven weeks ago," he said.

The ACT set a record for the number of tests it collected on Saturday, after capacity was boosted at government-run clinics at the weekend.

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More than 3300 tests were collected at government-run clinics on Saturday, while private providers collected 2638 samples.

More than 1 in every 100 Canberrans had a test sample collected on Saturday, and authorities remained confident most results were being returned within 24 hours.

Mr Barr said the testing rates were the equivalent of NSW testing 125,000 people in a day. Almost 127,000 people were tested in NSW on Saturday.

"It's a very big number for a jurisdiction of our size," he said.

Mr Barr also ruled out cancelling already booked vaccination appointments to free up staff to boost testing capacity.

"This is one of the workforce challenges that we are balancing here, but we've been able to surge up our testing capacity across the territory in partnership with private pathology as well to get to nearly 6000 tests yesterday," he said.

ACT Health had earlier flagged the vaccination roll out could be affected if the outbreak worsened, as it would drive up sustained demand for testing.

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