The National Gallery of Australia site.

ACT Health authorities suspect a visiting Sydney man that has caused alarm from a trip to Canberra after testing positive has possibly not spread COVID-19.

The man spent more than an hour in the National Gallery of Australia while observing the Bottcielli-Van Gogh exhibition, followed by a visit to the gift ship before later attending the Via Dolce Pasticceria restaurant in the nation's capital over the Queen's birthday Monday.

ACT chief minister Andrew Barr said investigations are underway over the gallery's ticketing system and the government's contract tracing will be examined from the check-in CBR app, but in addition he has asked the public to remain "vigilant" to the mildest symptoms.

It will be the first time authorities will be calling on data from the check-in CBR app - in response to a real case after practicing possible scenarios previously - to know how many people need to be contacted.

"It is a strong reminder that COVID-19 remains a risk," Mr Barr said.

"It is not the first nor will it be the last time during the pandemic there will be exposure sites and potential risk in the ACT."

A panel of health experts agreed it is better to be "overly cautious" than to under respond to this case.

However, authorities admit there is conjecture over what harm the man did from his visit.

The Sydneysider in his 40s had tested positive on a number of platforms, but negative on others.

"We are not 100 per cent sure it is a case, but we can't exclude the fact that it is," ACT chief medical officer Kerryn Coleman said.

"He is certainly not highly-infectious because otherwise we would not get results in this case.

"There are a couple of options that it may be an old infection or it may be reacting to some other virus that he has in his system."

The four locations from the man's visit will not be deep-cleaned days later as the virus does not last long on surfaces, but Dr Coleman added that Canberra's national gallery already has a "very rigid" Covid-safe plan.

Canberrans were quick not to take a risk after the Weston Creek Walk-In Centre reported that by early Friday morning (June 18) a greater increase for COVID-19 tests.

Numbers have also risen from 120 online declarations completed and the 100 calls to the ACT tracing centre.

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Ian Meikle, editor