Victoria could have Australia's first coronavirus reinfection, one of only a handful that have happened worldwide during the pandemic.
While health authorities are not certain, they are treating the Melbourne man's positive case as a reinfection, rather than him shedding the virus he first caught in July.
It was the only new case announced on Tuesday and Premier Daniel Andrews said health experts had looked at it in detail.
"Frustratingly ... they can't be satisfied to a pretty high standard that this is not a new positive case," the premier said.
"We are assuming, through an abundance of caution, that this person has got it for a second time.
"Perhaps we'll never actually know, but I think it's better to assume and be cautious.
"This is the best way to go."
There are no details on the man or whether he showed symptoms before his latest infection.
Mr Andrews also confirmed an inquiry into the state's flawed hotel quarantine program had asked him to answer some questions after Tuesday's extraordinary hearing.
"I would say it is an exercise for completeness' sake if you like. I'm not changing my evidence," he said.
Chief health officer Brett Sutton has been asked to provide a new statement to the inquiry after emails emerged contradicting his earlier testimony about when he knew the program was using private security contractors.
On Wednesday, Victoria reported no coronavirus deaths and only three new cases.
Melbourne's daily case average dropped to 6.2 and the mystery cases from October 5-18 also fell to 10.
The corresponding figures for regional Victoria remain steady at 0.4 and none.
The state's death toll remains at 817 and the national figure is 905. Victoria has had only one death in the past week.
All the new cases are in Melbourne, with two related to the northern community outbreak and the other under investigation.
There are 109 active cases in Victoria and the three regional positives remain the greater Shepparton outbreak from last week.
The Victorian government has also backflipped over its controversial call to allow a select group of spectators on course for the 100th running of the Cox Plate, following a public backlash.
Racing Minister Martin Pakula apologised after just hours earlier announcing up to 500 racing connections would be able to attend Moonee Valley for Friday night's Manikato Stakes and Saturday's Cox Plate.
The premier also apologised on Wednesday for the government's original announcement.
Asked if originally thought it was a right call, he replied: "Yes I did and it was the wrong decision to make and that's why it's not happening.
"For any inconvenience or any distress that's caused, I am very sorry."
Meanwhile, Mr Andrews says the state is well placed to bring forward a further easing of virus restrictions slated for November 2, including the reopening of retail and hospitality industries in Melbourne.
"It's very important that we see this thing off properly, so Sunday will be a day where we can have more to say," he said.
On Tuesday, the leaders of seven major businesses, including Wesfarmers, Commonwealth Bank, BHP, Orica, Incitec Pivot, Newcrest and Coca-Cola Amatil, sent a joint letter to Mr Andrews pleading for him to allow businesses to reopen.
Mr Andrews said in reply that he "very much" hoped to further ease restrictions on Sunday.
He rejected comparisons between restrictions on indoor religious gatherings and those at pubs, saying the latter were better regulated.
"Are people singing at the pub? I don't think so," he said.
Also on Wednesday, the state government announced that Melbourne cafes and restaurants will not need new planning permits for outdoor dining areas when they are allowed to reopen.
Australian Associated Press