The coronavirus situation in Hong Kong is “really critical”, with a record 100 new infections recorded on Sunday, the territory’s leader, Carrie Lam, said, as Melbourne became the first city in Australia to make wearing masks compulsory in response to a resurgent and aggressive outbreak there.

Hong Kong was held up months ago as a model for its success in keeping down Covid-19 cases in the crowded city-state of 7.5 million people, but its caseload – although still low by European and American standards – had grown by a third in the past fortnight to nearly 1,800. Lam has shuttered bars, gyms and nightclubs in the past week and on Sunday announced new guidelines including mandatory mask-wearing indoors.

“I think the situation is really critical and there is no sign the situation is being brought under control,” she told reporters.

Melbourne, too, was thought to be on the way to eradicating the virus a month ago but has since reinstated a full lockdown in response to record daily increases in infections. On Sunday, the state premier, Daniel Andrews, said everyone in Australia’s second-largest city as well as an adjacent area would have to wear a mask or a face covering from 11.59pm on Wednesday.

It is the first time masks have been made compulsory in the country, with a threat of a A$200 (US$111) fine for non-compliance. Andrews announced a further 363 cases of Covid-19 as he introduced the face-coverings requirement on Sunday, marking a fortnight of triple-figure rises in new daily cases in and around the city.

“We’re going to be wearing masks in Victoria and potentially in other parts of the country for a very long time,” he said.

“There’s no vaccine to this wildly infectious virus,” he said. Masks are “a simple thing, but it’s about changing habits, it’s about becoming a simple part of your routine”.