Victoria will effectively close its border to South Australia for 48 hours after fragments of COVID-19 were detected in wastewater along two freight corridors.
Premier Daniel Andrews says a "hard border" will be in place from 11:59pm for two days, before a permit system is introduced on Sunday.
Under the hard border, only freight drivers and those with medical or emergency reasons, as well as people authorised by law, such as child protection officers, 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.
"These arrangements will not be in place a moment longer than they need to be."
Travellers to South Australia from Victoria have been required to quarantine for 14 days. The requirement is expected to lift on December 1.
Victoria, meanwhile, had open borders for the duration of the pandemic until now.
The permit system is still being developed, but it will allow essential and agricultural workers to cross the border, as well as people shopping for essential supplies, receiving medical care and those visiting for compassionate reasons.
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 by health authorities 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 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.
South Australia is in its first day of a six-day 'circuit-breaker' lockdown to stop a second wave of coronavirus.
Like Victoria's second wave, which resulted in more than 18,000 infections and hundreds of deaths, the outbreak spread from staff working at a quarantine hotel into the community.
No international flights are arriving into either state, although Mr Andrews has flagged the program will resume in Victoria shortly.
"Everybody who works in this program will either work for the Victorian government or be exclusively contracted for this purpose and this purpose only," he said.
"We will advance contact trace every single person who works in this program to work out who they live with, what those people do for a living.
"We don't want a situation where someone is sharing a house with an aged care worker that we think would be an unreasonable risk."
Mr Andrews also flagged frequent testing of staff working in the program.
Victoria recorded its 20th consecutive day with no coronavirus cases or deaths on Thursday.
There are three active cases in the state, with 17,161 people tested in the previous 24 hours.
Mr Andrews said the border closure would 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