South Australians will no longer be allowed to enter Victoria with the Victorian government closing the border from Thursday night.
The measure was announced after The Standard revealed COVID-19 fragments were found in Portland's sewage.
Premier Daniel Andrews said the government was taking decisive action to protect the achievement of Victorians in delivering zero cases for 20 days.
Mr Andrews said the temporary border controls would be in place at midnight on Friday.
"Victorians have worked too hard and given too much to allow anything to put at risk our goal of reaching COVID Normal by Christmas," Mr Daniels said.
"We'll do whatever it takes to keep Victorians safe."
"We know border communities have had an incredibly difficult year - and we don't take these decisions lightly.
"Our public health team will work closely with South Australia to monitor the outbreak and get the border back open as soon as it's safe to do so."
A permit system, which will allow emergency service workers, essential service workers, people completing agricultural work, seeking medical care or shopping for essential items will be in place from Saturday.
Check points will be set up on the Victorian/South Australian border.
Minister for Health Martin Foley asked Portland and district residents with symptoms to get a coronavirus test.
"We'll work with these communitiesto ensure they have accessto the thingsthey need, as we provide further details on border crossings in the coming days."
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