Mr Barr told ABC radio residents would be mad to travel to a Covid hotspot while numbers continued to climb.

"What we've been witnessing in NSW, the challenges that they're having to deal with, suggests that this would be the most concerning outbreak really this calendar year," he said.

There are now 36 cases linked to the outbreak that began in Bondi last week, while exposure sites continue to grow.

Mr Barr said while it was NSW responsibility to contain the virus, the ACT government had to plan for the reality that it might get out.

"There are a series of escalating scenarios in the ACT depending on the nature of the transmission here," he said.

"From short-sharp lockdowns or longer ones are all available, depending on the circumstances."

The warning came after a man who attended a function at the Kingston Hotel this week was deemed a contact of a positive COVID-19 case. NSW Pharmacy Guild president David Heffernan has since returned a negative result.

Earlier on Thursday, Deputy Prime Minister Barnaby Joyce was alerted to the fact he many be a close contact with another person who had tested positive.

Travel between Sydney and Canberra has not been banned, however, the latest health advice for ACT residents was to avoid travelling to Sydney unless "they absolutely have to".

NSW Premier Gladys Berejiklian has so far resisted calls a city-wide lockdown. Not commenting on whether it was the right decision, Mr Barr said the spread from Sydney's eastern suburbs to the south west was of concern.

Prime Minister Scott Morrison that he was confident NSW could control the outbreak.

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"New South Wales, I have no doubt, has the gold standard contact tracing system not just in Australia but in the world," he told Sky News.

"My fellow Sydneysiders can feel very confident that if anyone can get on top of this without shutting the city down it is the NSW government."

Epidemiologist Mary-Louise McLaws said a short sharp lockdown should have happened last week.

"Epidemiologists who have worked in outbreaks have the adage that we go in early, we go in hard and get out rapidly because we know it really hurts," she told Nine.

Border closures and other restrictions have been slapped on Sydney by other states ahead of school holidays in NSW.

- with AAP

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