The Delta coronavirus strain that has caused devastation in India and the UK has been detected in Victoria's latest outbreak.
The state's Chief Health Officer Brett Sutton told reporters on Friday the strain has not been linked to any of the other cases in the current outbreak, which means it is not related in terms of transmission.
"It has not been linked to any sequence cases across Australia, from hotel quarantine or anywhere else," he said.
While the Delta variant has been found previously in Australian hotel quarantine, this is the first time it has emerged in community cases.
Prof Sutton said the B1.617.2 variant has "very high transmission potential", and while there is little information about the severity of illness it causes, there are anecdotal reports it causes more severe illness in children.
"It is a variant of significant concern," Prof Sutton said.
Genomic sequencing from two cases in a West Melbourne family has revealed they are infected with the Delta variant.
It remains unclear how those family members, who also travelled to Jervis Bay in NSW, contracted the virus, but the possibility that they picked up the virus in New South Wales has not been ruled out.
The family has more than 300 close contacts, many of them at North Melbourne Primary School, and authorities suspect there has been transmission between two grade five students at the school.
Some of the close contacts also live in public housing towers, which became a key focus a year ago early in Melbourne's second lockdown.
"It is a concern that it is not linked to other cases, but we are chasing down all those primary case contacts for that family and looking into where it might have been acquired," Prof Sutton said.
Three of Friday's four new cases are linked to the family, taking that West Melbourne cluster to seven.
No-one has been hospitalised locally as yet with the Delta variant, which is a "cousin" of the Kappa strain that sparked the rest of the current Melbourne outbreak.
Meanwhile authorities are re-examining virus samples and redoing genetic sequencing where possible across Australia, in an attempt to find out how the Delta strain has spread.
Prof Sutton says this included re-examining strains in quarantine, and cases linked to maritime, airline, and diplomatic travellers.
The total number of infections in the outbreak has reached 65, but one person has recovered, taking the number of active cases to 64.
Of more than 6000 primary close contacts across the Melbourne outbreaks, 90 per cent have returned negative test results.
Just under half-a-million people have been tested since Monday of last week.
It comes after Thursday's confirmation that two earlier cases in the outbreak had been ruled false positives.
Those two cases were part of the justification for the extension of a lockdown for Greater Melbourne by another week.
Victorian Acting Premier James Merlino said despite this, there were no immediate plans to shorten the lockdown.
"Our answer on that hasn't changed and nor should it," Mr Merlino said.
But the opposition leader Michael O'Brien said if the lockdown had been extended based on false positive cases, then Victorians deserved an easing of restrictions.
"You can't protect public health if you lose public trust", he said.
On Friday, the Health Department said there are still eight cases of transmission through incidental contact, plus five exposure sites where the virus infected people who do not know each other.
Also on Friday, Mr Merlino said the Commonwealth would try to help Victoria meet an increased vaccine demand.
Victoria wants to double the number of AstraZeneca doses available to GPs, and an extra 100,000 Pfizer doses from mid-June for its public vaccination sites.
There are now 358 sites where exposure to the virus may have occurred.
Meanwhile, the federal and Victorian governments have signed a memorandum of understanding to build the state's stand-alone quarantine site, with Avalon appearing to be Canberra's preferred location.
There were 49,439 tests in the 24 hours to midnight on Thursday and a record 24,169 vaccine doses.
Australian Associated Press