Victoria has recorded no new local COVID-19 cases for the third straight day as authorities admit they may never get to the bottom of exactly how a hotel quarantine worker picked up the virus.
The state recorded no cases locally or in hotel quarantine on Saturday, with just under 15,000 tests taking place.
There are 21 active COVID-19 cases in the state.
Meanwhile, a ventilation review of all Victorian quarantine hotels has been initiated and face shields made mandatory among workers.
Emergency Services Minister Lisa Neville said the government would not "leave any stone unturned" in its quest for answers.
Other implemented changes include establishing "buffers" between family groups and other guests, and staggering food deliveries.
If advised by health authorities, Ms Neville also said she would install CCTV on all floors of every quarantine hotel.
The moves follow a hotel quarantine worker at Melbourne's Grand Hyatt testing positive for the infectious UK variant of coronavirus and a potential case of guest-to-guest transmission at the Park Royal.
Victoria's Chief Health Officer Brett Sutton acknowledged officials may be unable to determine how the 26-year-old man working at the hotel - one of three used to quarantine Australian Open arrivals - came to be infected.
"I'm not sure we'll find out exactly what happened," Prof Sutton said. "Issues that have been identified in Queensland might be at play here."
Prof Sutton was more certain of the transmission source of the Park Royal case, in which a woman staying next door to an infected family of five tested positive despite having no contact.
"It does look like it's a case of doors being opened at the same time," he said.
All 17 household and social primary close contacts of the infected hotel quarantine worker have returned negative tests and remain in isolation.
Prof Sutton said it was "early days" and officials wouldn't feel comfortable the state was in "safe territory" until the end of the 14-day incubation period.
A further 1,129 primary workplace and exposure site close contacts have been identified and are being placed in isolation.
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