Melbourne could be on the brink of another lockdown after Victoria 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, with an official SA report on Wednesday suggesting he caught the virus from aerosol transmission in an Adelaide quarantine hotel.
But a definitive link between the Wollert man and the Whittlesea 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 positive cases have been out in the community.
He 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," Prof Sutton said.
There are now more than 60 exposure sites across Melbourne and regional Victoria, including the MCG and Marvel Stadium.
An infected person attended the Collingwood-Port Adelaide game at the MCG on Sunday, while one also attended the North Melbourne-Essendon clash at Marvel Stadium later that same day.
Spectators who sat in the same area as the cases 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.
An 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.
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.
Authorities are urging people to get vaccinated, with Mr Merlino again flagging the possibility of expanding eligibility criteria in Victoria.
It comes after a number of restrictions were introduced for Greater Melbourne on Tuesday evening, including gathering limits of five people at homes and 30 in public and mandatory masks indoors.
South Australia has slammed its border shut to Greater Melbourne on Wednesday, while NSW and Queensland are urging residents to reconsider travel to the city and parts of regional Victoria.
Victoria last entered a five-day "circuit breaker" lockdown in February, when the virus escaped hotel quarantine and infected 13 people.