Victoria will close its border with South Australia for 48 hours, after fragments of COVID-19 were detected in wastewater along freight corridors.
Premier Daniel Andrews on Thursday announced a "hard border" will be in place from 11:59pm for 48 hours, before a permit system comes into effect from Sunday.
Under the hard border, only freight drivers and those with medical or emergency reasons, urgent animal welfare or as authorised by law will be able to cross the border.
More than 300 police will patrol the Victorian side of the South Australian border, from Mildura down to Portland.
"There's simply no way that we can have people who ought not be leaving their home in South Australia doing so and then travelling to our state, not at this time," Mr Andrews told reporters.
The permit system will allow essential and agricultural workers to cross the border, as well as those shopping for essential supplies, receiving medical care or visiting for compassionate reasons.
The premier said the restrictions will not be in place "any moment longer than they need to be".
It comes after fragments of the virus were detected in untreated wastewater taken from Benalla and Portland treatment plants on Tuesday.
The preliminary positive test results, which were received on Thursday, were "unexpected and concerning", as there are no residents of either area known to have had a recent coronavirus illness or diagnosis.
Victoria's Deputy Chief Health Officer Allan Cheng said the viral fragments could either be from an active case, or a recovered case who may still be shedding the virus.
"It's always difficult to interpret," he said.
"It is possible that a recovered case for Melbourne has visited these areas."
Residents of Benalla and Portland, as well as anyone who visited between November 15 and 17 with "any symptoms at all" are being urged to get tested and to isolate until they get their result.
The move comes as Victoria recorded its 20th consecutive day with no coronavirus cases or deaths.
There are three active cases in the state, with 17,161 people tested in the previous 24 hours.
Mr Andrews said the border closure will not impact plans to further ease restrictions in Victoria on Monday.
The state's virus death toll stands at 819, with the national figure 907.
© AAP 2020