The finishing touches are being put on a glitzy show at the Sydney Opera House, as the venue prepared to host an opera crowd for the first time since March.

'The Merry Widow' will open on Tuesday to masked audiences up to 75% capacity, in a sign of hope for a performing arts industry crippled by the pandemic, artistic director Lyndon Terracini said.

"Walking back into the theatre was a very emotional time for everyone involved," he said. 

"I think throughout this year, other opera houses will be opening very soon and people will be coming back to the theatre with a sense of hope."

Thanks to Australia's success in suppressing the virus, crowds inside venues - including the Sydney Opera House - have been permitted in the country's most populous city for months.

But even as the performers readied for their opening night, an outbreak in the city forced officials to tighten restrictions - including a new mandate on mask-wearing on public transport and in many indoor settings.

New South Wales, which has close to 200 active Covid-19 cases, will levy a A$200 (€126) fine for not wearing a mask with the rule being enforced from Monday.

NSW Premier Gladys Berejiklian also announced reductions to gym classes, banned dancing and singing at night clubs while restricting numbers at weddings, funerals and places of worship.

However, the five-day cricket test match between Australia and India, scheduled to begin on Thursday, will go ahead with attendance at 50% capacity, she said.

The latest outbreak first emerged in December in Sydney's northeast suburbs but has since sparked other clusters, including in Melbourne, Victoria.

Julie Lea Goodwin, who leads the show along with Alexander Lewis, said she was thrilled to be back performing but after a nine-month hiatus the uncertainty of the pandemic still loomed. 

"I have no idea what's ahead," Ms Goodwin said. 

"I think that Australia is doing an unbelievable job... but it's just going to be a process for the next year, I'd say, or longer."

Australia has recorded over 28,400 cases of the virus and 909 deaths linked to Covid-19 in a population of about 25 million.