A Victorian hotel quarantine worker has tested positive to COVID-19, prompting the state government to reintroduce masks indoors and reduce the size of private gatherings.
Premier Daniel Andrews held a late-night press conference on Wednesday to confirm the 26-year-old man, who worked in the Australian Open hotel quarantine program, had tested positive to the virus.
The man worked his last shift at the Grand Hyatt in Melbourne's CBD on January 29.
He returned a negative test result at the end of the shift but later developed symptoms, got tested and returned a positive test on Wednesday evening.
The worker has been transferred to a health hotel and his close contacts are isolating.
"This is one case. There's no need for people to panic. There's no need for people to be alarmed," Mr Andrews told reporters.
He said out of an "abundance of caution" the state would reintroduce compulsory masks indoors from Thursday.
Private gatherings will also be reduced from 30 people to 15.
A number of exposure sites have been listed on the Health Department website, located mainly in Melbourne's southeast.
Victoria's Testing Commander Jeroen Weimar said the man provided "outstanding" information to contact tracers.
"We are continuing to talk to him tonight to extract as much information as possible about his movements, to ensure we have an accurate view about his contacts and his exposure sites," he said.
It comes just hours after the more infectious B117 coronavirus variant, first detected in the UK, spread between two separate groups of returned travellers at Melbourne's Park Royal Hotel.
Victoria's Police Minister Lisa Neville, responsible for overseeing the revamped hotel quarantine program, said genomics had shown the infections were identical.
"That means it's as if they have been in the same room together," she told reporters earlier on Wednesday.
One of the groups, a family of five who are all now infected with the virus, arrived from Nigeria on January 20 and tested positive four days later.
A fellow returned traveller in the opposite room, who restarted her 14-day quarantine stint after her husband arrived on January 16, twice tested negative before returning a positive result on January 28.
Questions remain over how the woman in her 60s became infected, given her partner's day three and 11 swabs were negative.
Ms Neville said security footage outside the rooms had been reviewed, with no indication of any breach of protocols by the families or staff during their stay.
"The viral load in the room of the family of five ... was so high that just even opening the door to pick up your food has seen the virus get into the corridor," she said.
The infected woman remembers opening her door at the same time as the room opposite but she has not been able to pinpoint the exact date or time.
Deputy Chief Health Officer Melanie Van Twest said authorities believe the potential room leak stemmed from the family's collective infectiousness in combination with the potent UK strain.
"This might be a Swiss cheese line of holes where everything has lined up to create this particular event," she said.
The hotel's ventilation system will be reviewed, although Ms Neville said an earlier report had found no air was being shared between rooms or into common spaces.
"It's probably unlikely to have been the ventilation system in this case," she said.
All positive cases have been moved to a health hotel, while the husband of the infected woman has been moved to another room.
Some 100 hotel quarantine staff members and 37 returned travellers who have completed their 14 days on the effected floor are now self-isolating at home. None have tested positive so far.
It comes after Victoria reached 28 days without any new local cases, widely regarded as the milestone for community elimination of the virus.
Australian Associated Press