The latest infection was confirmed just before Acting Premier James Merlino gave an update on Wednesday morning.
He said the next 24 hours would be critical as the government considered further restrictions.
"I cannot rule out taking some further action, but we will update people as soon as we know," Mr Merlino said.
"It's fair to say the 24 hours are going to be particularly critical.
"It's good that everyone (cases) is linked at this stage, but we are concerned about the number and also about the kind of exposure sites."
The number and spread of exposure sites around Melbourne has grown rapidly, from 10 on Tuesday to 50 on Wednesday morning, including the MCG.
Health Minister Martin Foley said there was "concern" about three inner-suburban entertainment venues on Saturday night and Sunday morning that were tier-one exposure sites - Three Monkeys and the Somewhere Bar in Prahran, and South Yarra's Circus Bar.
The new cases come as one of the infected people from the growing coronavirus cluster in Melbourne's north attended an AFL match at the MCG at the weekend, prompting a government health warning.
The infected fan was at the Collingwood-Port Adelaide game at the MCG on Sunday, sitting in zone 4, level 1 of the Great Southern Stand at the Punt Road end.
Spectators who sat in the same area are being contacted by Victoria's health department using data from tickets and QR codes. They will have to get tested and isolate until they receive a negative result.
Others who sat in the surrounding area are being told to go for testing if they develop symptoms.
"Further reviews of CCTV footage will be undertaken to determine if the advice to test and isolate will be broadened beyond these areas," the department said in a tweet on Wednesday morning.
Authorities sought to reassure people who attended the gaming, saying the AFL and Melbourne Cricket Club had "prepared for this situation in multiple simulations".
Mr Foley said of 301 primary close contact identified as of Tuesday night, 80 had returned negative tests.
In response to the unfolding outbreak, home gatherings are now limited to five visitors a day and public gatherings restricted to 30 people until at least June 4.
Masks are mandatory indoors for people 12 and over, but can be taken off for eating, drinking and exercise.
Schools and workplaces remain open, as are shops, cafes, restaurants and pubs and existing density limits apply.
The restrictions apply to Greater Melbourne, or people travelling into regional Victoria.
"This is about giving our contact tracers the time that they need to track this matter down and get on top of it," Mr Merlino said.
A public health advisory panel will soon decide if further restrictions are needed for large events including AFL games and the RISING festival, which is due to begin on Wednesday.
The Western Bulldogs have already had their weekly preparations interrupted, with the AFL club's players and staff forced into isolation after an employee attended an exposure site.
They reported on Wednesday morning that all their tests were negative.
Sports Minister Martin Pakula said the AFL would make an announcement on Wednesday about crowds at Melbourne games.
Genomic sequencing shows the outbreak is linked to the case of a Wollert man, who contracted the virus in South Australian quarantine earlier this month.
The man, in his 60s, could be the possible "source case" for the City of Whittlesea outbreak, although a definitive link is yet to be established.
Queensland, Western Australia and South Australia have significantly tightened their borders to Victoria while stopping short of slamming them shut for most travellers.
State Opposition Leader Michael O'Brien said Victorians were on edge.
"The government (has) to learn the lessons of this stuff-ups in the past and fix it for now because we cannot afford another lockdown," he said.
