In his daily update, the chief health officer Brett Sutton said the "weak positive" case revealed on Friday morning was "most likely a false positive or reflects a historic infection".
The patient, a woman in her 80s, had since returned two negative tests, Professor Sutton said.
"The case is being reviewed by the Expert Review Panel tonight," he said.
"Precautionary public health actions are in place, including isolation of the case, quarantining and testing of primary close contacts and secondary close contact tracing. These actions will be reviewed following the panel assessment."
Professor Sutton has again urged residents and recent visitors to Portland and Benalla to get tested for COVID-19, after traces of the virus were found in the towns' wastewater.
VICTORIA has recorded no new cases for a 21st consecutive day after locking down its border with South Australia.
The Department of Health and Human Services confirmed there were no new cases or deaths reported on Friday morning.
But further work was being done on a possible case, the department said on social media.
"There is a possible case reported that is under investigation; the initial test result is a weak [positive]," DHHS said.
"Further testing today, [including] review by an expert panel. The case may be a false [positive] or historic infection. Precautionary public health actions in place as investigations continue."
More than 18,000 Victorians got tested yesterday. There were two active cases remaining.
As announced on Thursday, viral fragments of the virus were found in Victorian sewage testing along freight routes this week.
It prompted premier Daniel Andrews to lock down the SA border.
None of the fragments revealed by authorities have been found in central Victoria, so far. They were found in Portland and Benalla, where residents were urged to get tested.
The hard border will stay in place until a permit system begins over the weekend.
South Australia is now several days into a snap lockdown authorities hope will be a "circuit breaker".
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.
It's the first time Victoria has shut its border to any state during the pandemic.
- With AAP
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