Contact tracers might never find the source of more than 20 cases of COVID-19 in the ACT, as health authorities continue to grapple with mystery cases in the outbreak.
Chief health officer Dr Kerryn Coleman said the lockdown had been effective in the ACT but it would need to continue.
Chief Minister Andrew Barr said the government would "give it our best shot" to bring the ACT back to no new COVID-19 cases in the community.
Dr Coleman said health authorities would need to closely monitor the number of unlinked cases of COVID-19 in the ACT and the number of cases infectious in the community as the outbreak continued.
"The proportion of cases which have been at least partially infectious in the community has also risen slightly over the last week," Dr Coleman said.
The ACT reported 13 new cases of COVID-19 to 8pm on Monday, seven of which were household or close contacts of existing cases.
However, at least eight of the cases were infectious in the community for at least part of their infectious period.
While 245 cases in the ACT outbreak have been linked, there are 29 which have no known source.
Dr Coleman said she was unsure whether links would ever be identified for 23 of those mystery cases.
The chief health officer said health authorities needed more time to contain the outbreak, but the lockdown had kept the effective reproduction rate of COVID-19 in the ACT very low.
Most cases of COVID-19 in the ACT were not passing the virus on to another person under the current lockdown settings, Dr Coleman said.
However, she said, with the number of unlinked cases and cases active in the community, the lockdown would need to continue.
"The best decision I think we can make is to extend the lockdown for another two weeks to help keep the ACT safe," Dr Coleman said.
"Look, I know this has been a difficult time. And in this case, I think the end is going to be harder than the beginning. But I can assure you that this decision wasn't taken lightly."
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Dr Coleman said the ACT's initial one-week lockdown, which has now been extended twice, had been implemented based on the experience of other jurisdictions attempting to contain outbreaks.
"At the end of one week, we had actually seen a massive number of exposure sites and cases were very much rolling through, so we knew we had to extend," Dr Coleman said.
"We think we have the settings right, which has allowed us to give that little tiny bit of relaxation for people for our mental and our own wellbeing. But we need more time."
Dr Coleman told a press conference on Tuesday ACT Health was still attempting to contact a newly identified case from an outbreak at the Ainslie Village social housing complex, but had been unable to do so.
Two new cases were identified after all residents at the site were tested following the identification of a positive case who had visited the site.
An ACT government spokeswoman confirmed on Tuesday afternoon the case had been informed of their positive result and would be supported to isolate by ACT Health.
"The health and welfare of that person, along with that of the community, remains the priority," the spokeswoman said.
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