The confirmed case is a returned diplomat currently in quarantine. Two close contacts are also in quarantine.

Chief health officer Kerryn Coleman said the case was a low risk to the community.

"The diplomat travelled with his wife and there was a driver who drove them from the airport to the ACT, both [close contacts] are and will continue to be in quarantine," she said.

The man in his 30s arrived at Sydney International Airport on November 15 before he travelled to Canberra where he has remained in isolation.

Health Minister Rachel Stephen-Smith has also announced the ACT will remain in a public health emergency until February, as the country continues to grapple with the pandemic.

"The public health emergency declaration empowers the chief health officer to make directions and make decisions to protect Canberrans in the face of this global pandemic," Ms Stephen-Smith said.

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It is the first time in 26 days a coronavirus case has been detected in Canberra, with the city marking almost three weeks without an active case.

It is the third diplomat to test positive to the virus in the ACT.

Diplomats are exempt from hotel quarantine as Australia is required to provide them freedom of movement and travel under the Vienna Convention. Diplomats are not required to isolate in a hotel but are advised to do so at home.

The news came days after health officials began screening flights from Adelaide as a cluster in the city's north continues to grow.

As of Tuesday evening seven passengers had been tested for COVID-19 and sent into isolation in the ACT as a precaution.

Dr Coleman said the roadmap out of COVID-19 restrictions which was planned to be released next Friday will be delayed as the impact of the South Australian outbreak on the ACT needs to be better understood.

"This becomes very challenging for us as a community," she said.

"We have to work out what level of restrictions with business and community will enable us to live with a COVID safe normal moving forward and still be able to adequately and appropriately respond and dampen down outbreaks when they occur.

"We have and will continue to take a cautious and measured approach to ensure we keep the community as safe as possible moving forward."

NSW Premier Gladys Berejiklian has followed the ACT in advising residents to avoid non-essential travel to South Australia as the Parafield cluster grew to 20 cases.

Ms Stephen-Smith urged anyone who had been to South Australia in the past two-weeks to monitor the state's health advice.

"Until there is a vaccine available, we will be living with COVID-19," she said.

"We all need to take responsibility for that.

"I'm sure the South Australian situation has reminded people that everybody has a role to play, keeping up those good habits we've built up and being a little bit patient in terms of the restrictions that are still in place.

"We are doing very very well in the ACT but we don't want to put that at risk by becoming complacent."

There have been 115 infections in the capital. Three people have died and 111 people have recovered.

Almost 115,000 tests have been conducted including 729 in the past 24 hours.

More to come.