The Victorian government has warned the next 24 hours will be critical as health authorities try to contain the outbreak and consider further restrictions.
"I cannot rule out taking some further action, but we will update people as soon as we know," Acting Premier James 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."
Earlier on Wednesday, the health department announced an extra five infections, taking the total number to 14 from nine on Tuesday evening. But one more case was revealed just before Mr Merlino gave a briefing on the outbreak, expanding the Whittlesea cluster to 15.
Five of the new cases are workplace contacts and all the cases are the Indian variant of the virus.
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.
Meanwhile, the number of exposure sites around Melbourne has grown 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 now listed as tier one exposure sites - Three Monkeys and the Somewhere Bar in Prahran, and South Yarra's Circus Bar.
One of the infected people also attended an AFL match at the MCG at the weekend, prompting a government health warning. The 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 have been told to get tested and isolate until they receive a negative result. Others in surrounding areas are being told to go for testing if they develop symptoms.
Mr Foley said of the 301 primary close contacts identified as of Tuesday night, 80 had returned negative tests.
In response to the 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.
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.
The AFL is expected to make an announcement on Wednesday about crowds at Melbourne games.
State Opposition Leader Michael O'Brien said Victoria can't afford another lockdown.
Queensland, Western Australia, South Australia and the Northern Territory have tightened restrictions for people travelling from Victoria but stopped short of closing their borders.
For the latest updates and analysis tune in to The Project tonight 6.30 pm on 10 and 10 Play.