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 at a late night press conference.

Mr Andrews said it was possible the worker, who is also a CFA volunteer, had the highly infectious UK strain of the virus.

Premier Daniel Andrews has ordered a return to masks as contact tracers fan out across Melbourne. Photo: AAP

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 plan to have up to 75 per cent of workers back in the office from Monday has been paused.

A number of exposure sites have been listed on the Health Department website, located mainly in Melbourne’s southeast.

Mr Andrews said it was still not clear how the worker contracted the virus, given he had previously recorded negative test results while working at the quarantine hotel.

“We can’t particularly determine how it is the person became infected, given that they produced negative test results,” he said.

“All of that will become clearer with a combination of CCTV footage, genomic sequencing, further testing – all of that coronavirus detective work that already well and truly underway.”

Victoria’s Testing Commander Jeroen Weimar said the man provided “outstanding” information to contact tracers.

Perplexing mystery

“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.

The latest infections come after Victoria chalked up 28 COVID-free days and just hours after the more infectious B117 coronavirus variant, first

Victorian Police Minister Lisa Nevillesays no effort is being spared to find the transmission conduits. Photo: AAP

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 those 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.

-with wires