Acting Premier James Merlino said the Department of Health was notified the Oakleigh man, aged in his 60s, had returned a positive test on Thursday morning.
He was one of about 30 guests at a birthday party at his daughter's house in West Hoxton on Saturday night.
"My understanding at this point is that he's not yet being contacted by NSW's contact tracing team," Victoria's Covid commander Jeroen Weimar told reporters on Thursday afternoon.
"The reasons for that we'll work through in the coming hours."
More than 12 people at the event have now become infected, including the Melbourne father, who caught a return flight from Sydney Airport on Sunday afternoon.
Late on Tuesday evening, he started to feel unwell and was tested on Wednesday.
The JQ523 Jetstar flight and terminal four of Melbourne Airport have now been listed as tier one exposure sites along with a now-closed dry cleaning shop in Sandringham where he works.
"There are two people who work in that establishment, and there will have been customer contact," Mr Weimar said.
"I'm pleased to confirm there is a QR code system in place."
He added the infected man lives alone and his sole initial primary close contact is a work colleague, with their results expected back later on Thursday.
Authorities are also tracking down those aboard the flight from Sydney.
Victorian health authorities believe the man likely has the more infectious Delta variant, which has been circulating amid the growing Bondi cluster.
Health Minister Martin Foley said there was no suggestion the man breached any permit regulations, as West Hoxton was classed a green zone over the weekend.
It is unclear whether he is partially or fully vaccinated, though officials praised the man for immediately coming forward for testing.
"Today's case is a stark reminder of just how important it is to get tested," Mr Foley said.
In light of Sydney's worsening outbreak, Victoria will make the entirety of Greater Sydney and Wollongong "red zones" under the state's permit system from 1am on Friday.
Air passengers are still able to fly out from Sydney Airport as long as they have not visited any red zone areas.
But Mr Foley issued a stern warning for non-Victorian residents, with at least six put back on return flights to Sydney on Wednesday after arriving from banned areas without a permit.
"If you try to enter Victoria through the airport you can be fined and you will be sent back," he said.
Patrols are being ramped up along road borders across the Murray River as well, with police scanning number plates to identify potential Sydneysiders.
Despite the new Sydney-linked scare, Mr Merlino confirmed Victoria would proceed with the planned wholesale easing of COVID-19 restrictions from 11.59pm on Thursday.
Earlier, Victoria reported one new case over the 24 hours to midnight on Thursday among a primary close contact linked to the outbreak at Southbank's Kings Park apartment complex.
The man has been quarantining throughout his infectious period.
Another three new infections were detected among returned travellers in hotel quarantine, pushing up the state's active case total to 52.
Over the same span, some 22,800 test results were processed and 17,800 Victorians rolled up their sleeve to get a vaccine dose at state-run hubs.
Meanwhile, Treasurer Tim Pallas has revealed Victoria's economy took a $1.3 billion hit during its fourth lockdown.
Treasury estimates the first week of lockdown, which encompassed all of Victoria, cost $700 million, while the second week in metropolitan Melbourne cost $600 million.
