The Victorian government warns the next 24 hours will be critical as authorities work to contain a coronavirus outbreak that has grown to 15 cases.

Acting Victorian Premier James Merlino says the next 24 hours will be critical to avoid another lockdown after the state recorded six more locally acquired COVID-19 cases.

The new cases are close contacts of people who have already tested positive to COVID-19 as part of the City of Whittlesea cluster, which has grown since Monday to 15.

Five of the six new cases are linked to a finance firm in Port Melbourne where one infected person works.

"It's good that everyone is linked at this stage but we are concerned about the number and also about the kind of exposure sites," Mr Merlino told reporters on Wednesday.

"The next 24 hours are going to be critical.

"I cannot rule out taking some further action but we'll update people as soon as we know."

All 15 cases have the B1617 COVD-19 variant, which originated in India late last year.

Genomic sequencing shows the outbreak is linked to the case of a Wollert man, who contracted the virus in SA quarantine earlier this month.

But a definitive link between the Wollert man and the outbreak hasn't been established.

"The only missing link is the link to the Wollert case from the South Australian hotel quarantine," Chief Health Officer Brett Sutton said.

While confident authorities have "run down multiple generations of transmission", Professor Sutton is concerned about the amount of time that positive cases have been out in the community.

"There are potential exposures there that relate to some days ago that are yet to play out in new cases," he said.

Prof Sutton said any lockdown would be broad rather than confined to the northern suburbs.

"We've got significant spread of exposure sites at the moment so it's not something that we would look to at a postcode level," he said.

There are now more than 50 exposure sites across Melbourne and regional Victoria, including the MCG.

One infected person is a player for Moonee Valley FC, with the club warning anyone in the change rooms or at its Ladies Day event on Saturday to get tested then isolate.

Health Minister Martin Foley said there was also "concern" about three bars now listed as tier one exposure sites - Three Monkeys and the Somewhere Bar in Prahran, and South Yarra's Circus Bar.

An infected person also attended 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.

He said four new testing sites would be set up on Wednesday and praised more than 26,000 Victorians who were tested in the 24 hours to Wednesday morning.

A record 15,858 people received a COVID-19 jab at the state's mass vaccination hubs during the same period.

The trio urged anyone eligible to get vaccinated, with Mr Merlino again flagging the possibility of expanding eligibility criteria for vaccines in Victoria.

"We're just working through other details of what that looks like and getting advice from public health, but we are keen to explore what we can do about increasing eligibility," he said.

It comes after a number of restrictions were introduced on Tuesday evening, including gathering limits of five people at homes and 30 in public and mandatory masks indoors.

The restrictions apply to Greater Melbourne, or people travelling into regional Victoria.

Queensland, Western Australia, South Australia and the Northern Territory have tightened restrictions for people travelling from Victoria but stopped short of closing their borders.

Victoria last entered a five-day "circuit breaker" lockdown in February, when the virus escaped hotel quarantine and infected 13 people.