Victoria's three cases on Sunday were all linked to the same Melbourne restaurant, though the source of the infection is still under investigation.

Dr Young has stopped short of closing Queensland borders to Victoria or to the whole of NSW, where masks have been made mandatory in key indoor settings in Sydney and surrounding areas.

"Victoria's own assessment of the situation is that the risk of spread is right across the state, not just in Melbourne".

Victoria Police spokeswoman Belinda Batty told the Herald Sun: "They were spoken to by authorised officers and informed they would be required to quarantine for 14 days as per current CHO directions".

"This is a very serious matter".

"We are still concerned about a case that occurred yesterday where we still haven't managed to find links in terms of the Avalon cluster".

Dr Young said a close contact of a Victorian case had already been identified in the Mackay region and a casual contact had been found on the Gold Coast.

Mandatory mask restrictions are in place for many venues across Greater Sydney as NSW works to contain COVID-19 outbreaks while avoiding harsh lockdown measures.

Victoria's border is now closed to all travellers from NSW. They have both since returned to their home of Goulburn in NSW.

And while the border restrictions remain unchanged, Dr Young also advised Queenslanders now visiting New South Wales and Victoria to consider returning home.

"If you do decide to go, you need to be aware of the potential consequences, such as border changes and not making it home to Queensland before hotel quarantine is imposed".

"The strategy very much in New South Wales is not to impose any burden on our citizens unless it is absolutely necessary but also to make sure we keep jobs and the economy, mental health, wellbeing, and a sense of normality moving forward as well as, of course, controlling the virus".

The NT recorded seven new positive cases on Saturday among Australians who came in on repatriation flights - one from London and six from Chennai, India.

More than three-quarters of Queensland's cases of SARS-CoV-2 have been acquired overseas.

Victorian Premier Daniel Andrews adopted a very different strategy.

Andrew Bird, 31, is in country Victoria, visiting his family for the first time in over a year.

More than 26,000 swab tests have been conducted on hotel quarantine staff since Victoria resumed taking worldwide arrivals on December 7.

Western Australia reimposed a hard border with NSW on December 19 and then with Victoria on Thursday.