Authorities have warned that everyone is at risk from a resurgence of COVID-19, as Victoria recorded 66 new cases in the 24 hours to Friday.
Canberra has followed the example of Queensland and will now force anyone arriving from the Melbourne lockdown suburbs to stay in hotel quarantine at their own expense for two weeks.
In Queensland, Premier Annastacia Palaszczuk said police would make random checks on the backs of trucks looking for attempts to smuggle people across the border into Queensland.
Victoria was the only jurisdiction to record new cases on Friday. Its 66 cases followed 77 the day before and 72 the day before that.
Victorian Premier Daniel Andrews said numbers were showing early signs of stability but still unacceptably high and he could not rule out more lockdowns.
"It's very serious decision and a very challenging one but if it has to be made it will be," he said.
"We do need more time in order to to get a firmer hold on whether there's a positive trend there, but certainly to see these numbers relatively consistent is very pleasing."
Victoria has 442 active cases, with 23 people in hospital and six in intensive care. Across the rest of the country, just four people were in hospital with COVID-19 and one in intensive care.
Victoria is testing more than 24,000 people a day as it conducts mass testing in the lockdown suburbs, but Health Minister Jenny Mikakos said 10,000 people have refused tests for various reasons.
"Some people believe that coronavirus is a conspiracy or that it won't impact on them," she said.
In Canberra, deputy chief health officer Michael Kidd said the increase in numbers of people in hospital was "very concerning and reinforces the very serious risk of COVID-19".
"This is a warning for everybody in Australia and the importance of continuing to adhere to the measures in place to protect us," he said. "We are all at risk of resurgence of COVID-19."
Dr Kidd said it was too early to say things had stabilised but authorities were watching the Melbourne outbreak closely, and hoped to see numbers start to come down.
"I know this has come as a shock to many of you," he said, referring to the people now forced back into lockdown. "Please know that the rest of our country supports you and we are grateful to you for doing all you can to stop the spread of COVID-19 ... You are not only protecting your own family and friends, you are protecting the rest of the country."
Ms Mikakos said she had been riddled with anxiety from the start of the pandemic, and had sought explanations in recent weeks about "what had gone wrong".
The genomic study this week suggested a single source for the new surge in cases, possibly even a "super-spreader".
Victoria's deputy chief medical officer, Annaliese van Diemen, said authorities were investigating the possibility that a single very infectious person had visited multiple locations and sparked the outbreak, or alternatively that there had been multiple introductions from overseas.
"We don't have a single super-spreader per se, but it is one of the epidemiological theories that is being looked at by our team of very experienced epidemiologists who are trying to work through the data that we're getting."
Dr Kidd said super-spreaders had not been identified to date in Australia, but had played a part in the spread of the SARS coronavirus.
Melbourne is now making up almost the entire caseload of new cases anywhere in the country.
Canberra has had no new cases in four weeks, with the last case recorded on June 6.
From next Friday, July 10, Canberrans are allowed into Queensland if they sign a declaration that they haven't been to a lockdown suburb in the previous two weeks.
To date 104 people have died.