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.
"We don't think this is about ventilation," she said on Saturday.
"We rejected a number of hotels ... and the ones that we have used throughout this program are hotels that do not share air between rooms and into common areas."
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."
Health Minister Martin Foley on Sunday said more than 1100 people are now in isolation as a result of the man's COVID-19 diagnosis, with more than 70 per cent tested and returning a negative result.
That includes 21 primary close contacts, all of whom have tested negative.
"They are the real heroes of this response over the past few days - stepping up, doing the right thing," Mr Foley told reporters.
Some 50,000 tests have been conducted across Victoria since the man's COVID-positive status was revealed late on Wednesday.
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.
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.
Mr Foley also said Victoria would encourage returned travellers departing hotel quarantine to take a "day 16" COVID-19 test, two days after their departure. Returned travellers in the NSW system are now asked to do so.