Six new COVID-19 cases have been detected to bring the cluster total to 15.
Addressing media this morning, Mr Merlino said the next 24 hours would be “critical”.
“I cannot rule out taking some further action, but we’ll update people as soon as we know,” he said.
“These cases so far are all linked, and that’s a good thing, but we are concerned about the number (of cases) and the kind of exposure sites.”
However, Mr Merlino said he would not “foreshadow” what those future changes might be.
Melbourne has so far endured three lockdowns since the outbreak of the pandemic, including one that stretched for 17 weeks last year.
It exited its most recent lockdown, a five-day snap decision by the government, in February.
Victorian Health Minister Martin Foley said Melbourne’s immediate future was in the hands of residents, urging them to get tested to assist contact tracers.