It follows news that two of the infected people in the growing cluster attended AFL matches on the weekend.
"I want to be upfront with everyone this morning. I cannot rule out taking some further action, but we'll update people as soon as I know," Mr Merlino told reporters.
"It's fair to say that the next 24 hours are going to be particularly critical."
A cabinet meeting was scheduled for Wednesday evening.
An infected person attended the Collingwood-Port Adelaide AFL game at the MCG on Sunday, while one was also at the North Melbourne-Essendon clash at Marvel Stadium later that same day.
Mr Merlino said authorities had been able to quickly access the information needed to contact spectators who needed to get tested and isolate.
However, there is still more work to do, with authorities also looking at CCTV footage.
"We'll keep working through the interviews and further information, and there'll be more meetings over the course of the day," Mr Merlino said.
Hundreds of primary close contacts identified
The number and spread of exposure sites around Melbourne has grown rapidly from 10 on Tuesday to 79 on Thursday morning.
While the outbreak started in Melbourne's northern suburbs, locations in country Bendigo and Port Melbourne now feature among the exposure sites.
Health Minister Martin Foley said there was concern about three inner-suburban entertainment venues that were tier-one exposure sites - Three Monkeys and the Somewhere Bar in Prahran, and South Yarra's Circus Bar.
Mr Foley said of 301 primary close contacts identified as of Tuesday night, 80 had returned negative tests.
In response to the unfolding outbreak, home gatherings are now limited to five visitors per day and public gatherings restricted to 30 people until at least 4 June.
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.
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.
But Victoria's Chief Health Officer Brett Sutton said on Tuesday morning authorities were yet to establish a "definitive link" between the Wollert man and the outbreak.
He said on Wednesday that it was the only missing link in the cluster.
States and territories respond
Several states and territories have tightened their borders to Victoria while stopping short of completely slamming them shut for most travellers. South Australia, however, has announced the closure of its border to any travellers from Greater Melbourne from 6pm Wednesday.
The state will only allow essential workers and returning South Australians to cross the border but they must get three virus tests and isolate for 14 days.
Anybody from Greater Melbourne who travelled to South Australia in recent days must also get tested and isolate until they receive a negative result.
Meanwhile, NSW health authorities are urgently contacting members of a sporting club in the state's western Riverina region who attended an event in Victoria and may have come into contact with a confirmed case.
The club members from Tooleybuc attended an event in Cohuna on Saturday 22 May from 12pm to 5pm.
Anyone who attended is being asked to immediately self isolate and await further advice from NSW Health.
Earlier, Health Minister Brad Hazzard on Wednesday said "it might be wise" for residents to delay travel to Greater Melbourne.
"If you don't really need to go to Greater Melbourne at the moment, it might be an idea just to leave it for a few days until we get some clarity from the Victorian government as to what's going on," he told reporters.
Anyone in NSW who has been in the Greater Melbourne and Bendigo areas in the past 14 days must complete a form confirming they have not visited a Victorian venue of concern. They're also being asked not to visit residential aged-care or healthcare facilities, unless they are seeking treatment or for compassionate reasons.
With reporting by AAP.