Victoria's concerning new mystery virus case who is linked to Sydney's Northern Beaches cluster.
Genomic sequencing has confirmed the Melbourne man is infected with the same strain of the coronavirus that broke out in Sydney before crossing the border.
"It's from Sydney," Premier Daniel Andrews said, returning from leave ahead of Friday's national cabinet meeting.
"Exactly how it got from Sydney to here, that is less clear. But it's exactly the same strain as NSW authorities are battling now."
The man hadn't been to NSW or any high-risk exposure sites in Victoria.
Contact tracers are trying to establish whether there is a link to that mystery case and the 27 others from the Black Rock cluster, which has plunged 2,600 primary and secondary contacts into self-isolation.
Despite the origin remaining unknown, Mr Andrews insists the development is pleasing as it rules out the case stemming from hotel quarantine or having the more virulent strain of virus first detected in the United Kingdom.
"At the same time, though, any mystery case is a concern to us," he said.
"It means there's at least one other person who gave it to that mystery case."
The Vermont South man in his 30s is feared to have caught the virus either at Chadstone Shopping Centre on Boxing Day or the MCG Test the day after, with the venues deemed possible acquisition sites.
His case is the first of unknown transmission in Victoria since early November, turning up the heat on the state's improved test and trace program.
Thousands of impacted shoppers and cricket spectators are answering the call of health officials to get tested.
From 32,767 tests processed in the 24 hours to Thursday, there were no new local, interstate or overseas acquired cases.
Authorities have now contacted 1700 of the 8000 patrons registered as having been in zone five of the Great Southern Stand on the afternoon of 27 December.
They have been told to get tested and isolate until they return a negative result.
Thousands of shoppers who went to 11 Chadstone stores, including the newly-added Zara retailer, from 6am to 1.30pm on Boxing Day have been told to do the same.
More precise times for each store will be narrowed down and published by the health department on Thursday, the state's COVID response commander Jeroen Weimar said.
Meanwhile, about 400 border crossing exemptions have been granted to stranded Victorians wanting to come home from NSW.
Mr Andrews says an extra 250 staff are sorting through another 4000 outstanding applications and flagged permit settings could change as early as next week or the week after.
Victoria slammed shut its border to NSW on Friday night, with thousands unable to make it back before the deadline.
The premier believes those left languishing in limbo have been treated fairly, but conceded some may have a different view.
"I make no apology for the decisions that we've made," he said.
"I won't proxy off the protection of Victorians to decisions made by another state."
Mr Andrews returned from leave on Thursday to push for Victoria's pre-flight testing, mandatory flight crew quarantine and daily hotel quarantine staff testing policies to be adopted nationally.
He says those measures, championed by Victoria after its deadly second wave, are needed to ensure the UK's mutant coronavirus strain doesn't leak into the Australian community.
"If that gets in here and gets away from us, there will be no pulling it up," Mr Andrews said.
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