Dr Kerryn Coleman said she anticipated there would be an increase in cases in the ACT, following a rapid escalation in NSW.

The ACT has reported 53 cases of the Omicron variant but about half of those cases were acquired in NSW.

Hospital numbers remain low, with only three COVID-19 patients in Canberra hospitals. Nobody is in intensive care.

The new mask mandate will come into effect at 11.59pm on December 21 and people will be required to wear masks in indoor settings, including retail settings, public transports, indoor workplaces and hospitality venues (except when seated, eating or drinking).

Dr Coleman said wearing a mask would help to slow the spread of Omicron in the ACT.

"This is a commonsense step, we've been here before and I don't consider this too onerous for us to take to help us slow the transmission of the Omicron variant to protect our most vulnerable community members," she said.

COVID cases have spiked in the ACT, in the past week there were 91 new cases reported, which was up from 36 in the previous week.

Dr Coleman said it was likely the real number of Omicron cases was higher than reported, because genomic sequencing had not been finished or was able to be done on every sample.

But ACT health authorities will also no longer undertake genomic sequencing on cases for the sole purpose of determining whether it is the Delta or Omicron variant.

Dr Coleman said the strain would soon become the dominant variant in the ACT, comparing the Omicron situation in the territory to that in Newcastle.

She said while Omicron may not cause severe illness, it could overwhelm hospital systems because of high case numbers.

"[It is] less about numbers of cases in community and more about severity," she said.

"If the rapid escalation of cases is to such a level that the volume of cases are so big ... the percentage of cases going to hospital is concerning."

Alongside new masks requirements, new visitor restrictions have been implemented on Canberra aged care facilities. Under the new rules a resident will only be allowed to have five visitors a day.

There will be no daily limit on the number of visitors for end-of-life visits. Masks will be mandatory for staff and visitors.

There were 16 new cases in the ACT on Tuesday, with 124 active cases.

The mask mandate will be reviewed in January.

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ACT health authorities may also need to make changes to the rollout of booster shots if national cabinet decides to bring forward the waiting time for third doses.

Victorian and NSW health ministers have called for the five-month wait time to be shortened, however, ACT Health Minister Rachel Stephen-Smith would not be drawn on whether she thought the same.

She said she would wait for advice from the Australian Technical Advisory Group on Immunisation.

"ATAGI is the best placed authority to provide us with advice about the timing for booster doses and there are a lot of considerations that they need to take into account," Ms Stephen-Smith said.

"I can understand why people do want to see people having the strongest amount of protection from vaccination in a timely way ... [but] there are also other factors that ATAGI needs to consider when making those recommendations."

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