As the COVID-19 outbreak continues to grow in Australia, local authorities have sought to reassure Canberrans that no community transmission of the virus has been detected in the ACT.

Effectively, what that means is all of the ACT's cases fall into high-risk groups: those who have travelled interstate or overseas, or those who have been in close proximity to another confirmed case of COVID-19.

It can be a reassuring thought that you are unlikely to contract the illness by going about your daily business in Canberra, but the situation raises an inevitable question: how do we know the virus isn't spreading if we only test people who are already in the risk categories?

Canaries in the coalmine

The ACT has a testing rate of just under five tests per 1,000 people, and most of those tested are displaying symptoms and have either been overseas, travelled to a high-risk area interstate, or had close contact with someone who has COVID-19.

Testing was expanded over the weekend to include symptomatic people in high-density living situations like boarding schools and aged-care facilities. Hospital admissions showing COVID-19 symptoms are also tested.

But if the average person comes down with flu-like symptoms, they will likely not be tested.

Who should get tested? COVID-19's main symptoms are fever (above 38C), cough, sore throat and shortness of breath. Seek medical help if you: recently travelled overseas or on a cruise ship, and developed symptoms within 14 days of returning to Australia

recently travelled overseas or on a cruise ship, and developed symptoms within 14 days of returning to Australia were contact with someone who has COVID-19 and you developed symptoms within 14 days of last contact

were contact with someone who has COVID-19 and you developed symptoms within 14 days of last contact are a healthcare or aged-care worker with recent onset of symptoms, irrespective of travel history

are a healthcare or aged-care worker with recent onset of symptoms, irrespective of travel history live in a high-risk setting (e.g. aged-care or other care facility, boarding school, operational military setting, detention centre) and have symptoms

live in a high-risk setting (e.g. aged-care or other care facility, boarding school, operational military setting, detention centre) and have symptoms have links to an area where COVID-19 outbreaks have occurred in Australia, and have symptoms If you wish to be tested, call your GP, visit the drive-through testing centre at EPIC or the Weston Creek walk-in centre, or call Healthdirect on 1800 022 222. See the ACT Health website for more information.

It has prompted concern among many in the community and on social media that local transmission is going undetected in the general public.

But there are canaries in the coronavirus coalmine, the Government says, that would set off alarm bells on community transmission: healthcare workers.

"Our testing of healthcare workers who are symptomatic and have no known exposure … enables us to have a good indication of whether we're starting to see local transmission in the ACT," Health Minister Rachel Stephen-Smith said.

A drive-through coronavirus testing site opened at EPIC in Canberra's north last Friday. ( ABC News: Tom Maddocks )

Ms Stephen-Smith said a positive test of such a person would likely be the first sign of community transmission in the ACT.

"Or from someone who arrives in hospital with severe pneumonia or severe respiratory illness, who has no other explanation for it — those people are being tested as well," she said.

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All healthcare providers are recommended to be tested if they develop a fever or respiratory ailments — even if they have not been overseas or in contact with COVID-19 patients.

The Government says if they start seeing positive results among that group, it will be evidence of community transmission.

So far, all of those tests have been negative, leading to Government assurances there is no evidence of coronavirus spreading widely in the Canberra community.

So who has been infected?

Authorities say they will become far more concerned when new patients begin appearing without any links to known cases.

As of Tuesday afternoon, 39 people in the ACT had tested positive for COVID-19, and none of them fit that criteria.

Here's how authorities believe they got the virus:

Overseas travel: 32 people

Overseas travel: 32 people Close contact with a confirmed case in Canberra: 3 people

Close contact with a confirmed case in Canberra: 3 people Close contact with a confirmed case interstate: 4 people

Tim Newsome, who lectures in virology the University of Sydney, said Governments need to target testing to certain groups, as there are not enough testing kits to test everybody who gets sick.

"At the current state, these testing kits are in finite supply," Dr Newsome said.

"Testing the whole population is just not feasible."

Associate Professor Newsome said using healthcare workers as an indicator of whether there were more cases of COVID-19 in the community made sense.

"I think healthcare workers make sense from the perspective that you are protecting patients and also looking at community transmission," he said.

"You are learning about community transmission, and you are also, by identifying COVID-positive healthcare workers, able to remove them to quarantine and allow them to self-isolate and protect people who need treatment.

When will this change?

Changes to the testing regime have been put on the table for this week.

"We'll be removing the traveller component, but we're working on that at the moment," deputy chief medical officer Paul Kelly said on Q+A on Monday.

"There will be announcements about that over the coming days."

Ms Stephen-Smith said the Government was ready to change its testing requirements as the situation changed.

"We are very confident that the ACT and Australia have one of the highest testing rates in the world," Ms Stephen-Smith said.

"That testing criteria is under review every single day."