Victoria is on high alert after a man from NSW with COVID-19 travelled through the state and South Australia while infectious.
The man, who is a removalist, stayed overnight in Victoria on Thursday before driving to South Australia and later returning to NSW, where he tested positive for COVID-19 on Sunday.
Victorian Health Minister Martin Foley said NSW Health alerted the state's authorities late on Sunday night.
Contact tracers are working to establish the man's movements in Victoria, though it is believed he travelled from Sydney to Melbourne via the Hume Highway and worked at several homes in the city before travelling to SA.
"We would expect there will be exposure sites and we expect there will be people who require to quarantine," Mr Foley said.
Removalists are permitted workers under the state's border permit system.
Mr Foley said the risk of the virus spreading from NSW to Victoria was "very real".
"Our friends in Sydney are facing really challenging circumstances, and we wish them every success in their fight," he said.
"This is now the third incursion into Melbourne and Victoria as a result of this outbreak, we've run the other two down."
It comes after Victoria recorded its 12th day of no locally acquired cases and the state effectively shut the border to NSW and the ACT overnight, declaring them red zones under the travel permit system from 11.59pm on Sunday.
ACT Chief Minister Andrew Barr said it was "incredibly disappointing and frustrating" Canberra was classified the same as Sydney, despite recording no community cases of COVID-19 for more than a year.
But Mr Foley said the rationale behind the decision was "simple".
"The ACT ... is essentially surrounded by regional NSW," he said.
NSW reported 112 new local cases on Monday.
Victoria's acting chief health officer cited concerns about the risk of coronavirus transmission beyond Sydney and surrounding areas for the decision.
The border closure announcement was made just after 4pm on Sunday, giving little time for people to return on their existing orange zone permits.
But Victorian authorities had been foreshadowing a blanket NSW red zone declaration for days, encouraging residents to return home urgently.
Victorians in NSW will still be allowed to enter the state but must isolate at home for 14 days.
In addition, the Victoria-NSW "border bubble" arrangement will remain for local residents, though they must carry proof of address and stay within the bubble.
© AAP 2021