MASKS are back with a vengeance across Victoria after Premier Daniel Andrews announced a set of snap restrictions late on Wednesday night in response to a hotel quarantine worker testing positive for COVID-19.

The 26-year-old was working as a resident support officer at Melbourne’s Grand as part of the Australian Open quarantine program and health authorities were “assuming the worst”, with Mr Andrews saying it was likely he has the highly infectious UK strain of the virus.

The man’s close contacts have been contacted and instructed to isolate, and a number of locations in south-east Melbourne have been listed as exposure sites, including a Kmart store and a Bunnings Warehouse.

As of 11.59pm on Wednesday night, masks are now mandatory in all public indoor spaces, including workplaces, except when eating or drinking.

Victorians are still required to carry a mask at all times when they are outside the home, and people are strongly encouraged to wear masks when visiting homes as well as in public.

The limit on the number of people that can gather in a private home will be cut from 30 to 15, meaning the household members plus 15 visitors, not including babies under a year old.

The 75 per cent "return to work" cap for both public and private sectors scheduled for next Monday will be put on hold and the current cap of 50 per cent of the workforce in the office will remain in place.

But Mr Andrews said “there’s no need for people to panic” as authorities scramble to contain a potential outbreak.

“We've identified it relatively early,” he said on Wednesday night.

“I think people will absolutely step up to this in terms of knowing that they need to get tested and that they'll need to isolate until they get that negative result. If everyone's doing that, we'll absolutely get on top of it.”