Victoria's Covid-19 infections have spiked overnight, with health authorities confirming six new local cases overnight.
There are now 23 active cases in Victoria, with 15 of those linked to the current outbreak.
Six new cases were confirmed on Wednesday morning, with one of those - announced soon after an initial five new cases were revealed - to be included in tomorrow's tally.
It means a total of 10 new local infections were discovered on Tuesday.
More than 25,000 test results were received during the reporting period.
Speaking at a press conference on Wednesday, Acting Premier James Merlino revealed he was "very concerned" about the spike in cases, prompting further fears Melbourne could be sent into a snap lockdown.
A raft of new restrictions were introduced across Greater Melbourne on Tuesday, but Merlino warned the government hasn't ruled out taking further action.
"I want to be upfront with everyone this morning – I cannot rule out taking some further action, but we'll we will update people as soon as we know," he said.
"It's fair to say for the next 24 hours are going to be particularly critical so the message this morning is, it's good that everyone is linked at the stage, but we are concerned about the number, and also about the kind of exposure sites, and the next 24 hours are going to be pretty critical if we're going to make any further changes beyond the changes that we announced yesterday."
The current restrictions, which were introduced at 6pm yesterday, include:
• A limit of five visitors per day for private households
• A limit of 30 people for public gatherings
• Masks required indoors for everyone aged 12 and over — even in workplaces — unless an exemption applies.
So far 301 primary close contacts of confirmed coronavirus cases have been identified, with 80 of those returning negative test results.
A man in his 60s, who was among the five cases announced yesterday, is believed to be the earliest known infection in the new outbreak.
The man, referred to as case 5, developed symptoms on May 17, with authorities believing he may have been infectious for almost 10 days before presenting for testing.
Later in the day it was announced four family contacts of the man also tested positive. He is also believed to have transmitted the virus to the first person discovered in the cluster, a man in his 30s.
That man then passed the virus on to three close family contacts spread across two households in Whittlesea, including a man in his 70s, a woman in her 70s and a preschool-aged child.
Everyone in the new cluster has been confirmed as having the Indian variant of the virus.
Chief health officer Professor Brett Sutton said authorities have discovered "multiple generations of transmission" over the last two days.
"We remain concerned by numbers and the fact that we will have further exposure sites identified," he said.
Professor Sutton said the next 24 hours will tell authorities whether anymore primary contacts are out in the community, with the rest of the 300 test results to come through today.
"The exposure sites that they then generate, how many close contacts they might have and how long they have been in the community — they are all factors we need to consider," he said.
Genomic testing has found the new infections are "closely linked" to a case from a few weeks ago that originated in South Australia.
Earlier in the month a man tested positive to the virus after becoming infected while undergoing hotel quarantine in South Australia and then flying into Melbourne and returning to his home in Wollert.
Health authorities believe there could be a "missing link" between this case and the new cluster, though so far none of the new cases have been found to be linked to any exposure sites from the Wollert case.
Yesterday, chief health officer Professor Brett Sutton said the fifth case in the cluster could prove to be the missing link authorities are looking for, as the interview process was still ongoing.
"His initial recollections do not overlap with any of the wallet exposure sites, so there is no definitive link to that wallet sites even though we know there is a genomic link," he said.
Professor Sutton also warned there were clear "chains of transmission" playing out with these new Covid-19 cases.
He said the cluster show there was "clearly someone who was not identified" through recent contact tracing interviews, possibly because they were a casual contact.
"That is why I flagged public transport is a risk if you are on a packed carriage you do not know who you stand next to and you may be next to an infectious person," he said.
"I ask everyone to go back to those exposure sites on the website and, indeed, if they are going out for any gatherings, within limits from today and with 6pm restrictions in mind, go back to those exposure sites. If you have been to one of them, you should not go anywhere you should be tested."