The new variant, Delta B1617.2, has not been seen in Australia apart from a case in NSW last month.
Infectious diseases expert Dr Sanjaya Senanayake told Today “any unlinked case” was always a concern.
He believes the mystery infections could be due to a breach in a hotel quarantine system.
“It means there could be earlier generations of infections who are still out there in the community infecting other people,” he said.
“Presumably again, it is hotel quarantine somewhere where there has been a breach.
“We’ve seen, what, 17 breaches in the last six months so that’s the most likely scenario.”
There are seven cases linked to the West Melbourne outbreak infected with the Delta variant.
The school remains closed and more than 300 people are isolating.
Health officials yesterday said they remained concerned about the spread between children, including the effects the particular strain could have on kids.
“This strain is potentially more infectious … but any strain of COVID is bad,” Dr Senanayake said.
Victorian health authorities believe there is a possibility the family caught the virus on their road trip to Jervis Bay.
But NSW Health has said there is no evidence the family acquired it on the trip.