Another resident and worker at a coronavirus-stricken Melbourne aged care facility have tested positive, as health authorities discuss Delta strain leak "theories".
The two further COVID-19 cases linked to Arcare Maidstone in the city's northwest were reported on Sunday afternoon.
The 79-year-old resident, who has had both doses of the Pfizer vaccine, lives close to a 99-year-old woman and 89-year-old man who have previously returned positive tests.
Despite being asymptomatic, the resident will be taken to hospital.
The other case is a registered nurse who worked at the nursing home on Saturday, is also asymptomatic and has received a first COVID-19 shot.
Victoria's Deputy Chief Health Officer Allen Cheng was aware of Arcare's new cases but said authorities were still working through the details.
The two infections were not among Victoria's tally of two cases reported earlier on Sunday as authorities continue to search for the source of the state's worrying Delta strain.
Acting Premier James Merlino said both of those infections were primary close contacts of previous cases.
One is a worker in their 50s at Stratton Finance in Port Melbourne and was isolating for their entire infectious period.
It takes the Kappa variant-related Port Melbourne outbreak to 30 cases, with four previous "mystery" infections now linked through genomic and epidemiological investigations.
The other is a primary school teacher linked to the West Melbourne outbreak, which has infected 10 people with the Delta variant.
Professor Sharon Lewin from Victoria's genomic sequencing centre on Saturday revealed her "strong hypothesis" that it entered the community via hotel quarantine.
Professor Cheng said authorities were continuing to comb nationwide genomic sequencing data for answers.
"We have theories. We're working through those," he told reporters on Sunday.
"It is like a fingerprinting database. The people that have had the fingerprint, you can say they are not the source case."
With Melbourne enduring its second weekend of lockdown, Prof Cheng said authorities' main concern is tracking down "upstream" transmission.
"Given five per cent of Victorians have been tested in the last seven days, if there was a big outbreak going on I would expect to have picked it up," he said.
"That risk does fall over time but we are still concerned about that and that is where all our efforts are going."
He expects Melbourne will be able to ease restrictions on Friday.
"We don't want to be in this any longer than we need to. So if we can, we will lift it early," Prof Cheng said.
There are more than 6000 primary close contacts across the state, although Prof Cheng said 1900 would be released on Sunday after serving their isolation period.
Victoria's list of exposure sites remains above 350, with officials now focusing attention on a network of small businesses on levels four and five of 227 Collins St.
Viral fragments detected in wastewater also have authorities on alert for a possible undiagnosed infectious case in Melbourne's inner west and north.
Residents across 10 suburbs have been urged to get tested if they have symptoms following the "unexpected" recent wastewater sample detections.
"There are no confirmed cases in that area. There is one case that is a resident of that area but is in hotel quarantine," Prof Cheng said.
There were 29,816 tests in the 24 hours to midnight on Saturday and 19,940 vaccine doses administered.
Meanwhile, the Victorian government unveiled a $32.2 million support package for tourism and accommodation operators.
It includes $16 million for a further 80,000 regional tourism travel vouchers, $11.8 million for tourism grant top-ups and $4.4 million for Victoria's alpine industry on the eve of the ski season.