Victoria's concerning outbreak of the highly infectious Delta strain of Covid-19 has been linked to a returned traveller, though questions remain about how it leaked into the community.
The outbreak, known as the West Melbourne cluster, now has at least 14 confirmed cases linked to it, all of which are the Delta strain.
Victoria's Acting Premier James Merlino confirmed authorities have now found a genomic match between this cluster and a returned traveller who entered hotel quarantine on May 8 after arriving from Sri Lanka.
"While we have a genomic link, we do not currently have an epidemiological link, and further investigations are under way to see if we are able to establish any contact between the returned traveller and these families," he said.
"There's currently no definitive understanding of where transmission events may have occurred but we are investigating all possibilities from the plane to travel to hotel."
Victoria recorded two new locally acquired cases overnight, bringing the total number of infections in the state's outbreak to 85.
There was also one case confirmed in an overseas traveller currently in hotel quarantine.
It comes after 11 new infections were confirmed yesterday, with two of those reported late on Sunday.
'Four main theories'
The returned traveller tested positive to the virus the same day he arrived in Melbourne and was transferred from the Novotel Ibis quarantine hotel to the Holiday Inn health hotel on May 14 before being released from quarantine on May 23.
Investigations have so far revealed there were 24 people on the same plane as the case, with all crew and passengers testing negative.
Of the 268 staff who worked at the Ibis hotel on May 8 and 9, and the 360 staff at the health hotel, there were no positive tests identified. All 12 residents housed in the same Novotel Ibis have tested negative.
Deputy chief health officer Allen Cheng said this suggests there has been transmission either directly from person-to-person or indirectly via unidentified cases to the family and the community.
"The investigation is continuing but there [are] four main theories and some of them are more or less likely," Professor Cheng said.
"The first possibly is that the case transmitted to or was infected by someone — another passenger on the plane — and that person has gone on to infect someone in the community.
"The second possibility is that the case transmitted after he left hotel quarantine. The third is that the case transmitted to a staff member somewhere along their journey from the airport to transport to the first hotel to transport to the second hotel and then out to the community, and then the fourth possibility is that the case transmitted to a hotel resident who has then transmitted to the community after their quarantine stay."
The West Melbourne cluster has become a focal point in the race to contain Victoria's outbreak because of how fast the variant can spread.
Victorian chief health officer Professor Brett Sutton said the variant was "more dangerous" than others and that 75 per cent of people catch the strain when there is a case living with them.
He said that means the state is "racing against the clock" to contain it.
Melbourne lockdown still on track to lift
Melbourne's two-week lockdown is set to end at 11.59pm on Thursday (local time), with senior Victorian Government sources telling the Herald Sun they are very confident there will not be another extension.
However, the one thing that could force authorities to keep lockdown restrictions in place would be a significant rise in mystery cases.
Eased restrictions similar to those currently in place across regional Victoria are expected to be introduced in Melbourne once the lockdown ends and could include caps on private and public gatherings, density limits for venues and businesses, and mask-wearing indoors.
Prime Minister Scott Morrison added his voice to the growing number of people calling for an end to Melbourne's lockdown.
The PM said he was "hopeful" restrictions would be lifted "as soon as possible".
"I would be urging that we move toward lifting those restrictions as soon as possible," Morrison said.
"Hopefully [we] see Victoria opened again soon. Particularly for those parents who are having to keep their kids at home away from school. Kids have lost enough time out of school, over the course of the last 18 months."
It comes as Victoria's opposition leader Michael O'Brien blasted the ongoing restrictions, claiming families and businesses were suffering as a result.
He claimed the lockdown still meant that there's "a lot of Victorians who are pretty much living on the edge every day", and suggested the Government "doesn't understand just how difficult this lockdown is for so many Victorians".
"We can't keep going on like this," he said, accusing the Government of leaving one in three vaccines "gathering dust on the shelf".
"We need to protect this state and we need to do it quickly because we need to get out of this lockdown."