Residents of Perth and the neighbouring local government area of Peel will begin a three-day snap lockdown at midnight (local time) Friday after a new community COVID case was diagnosed.

The infected woman is a friend of the Victorian man who was confirmed with the virus on Friday morning, after returning home to Melbourne from 14 days in hotel quarantine in Perth.

He caught the virus from an infected couple while in the Mercure Hotel, in Perth’s CBD.

The man spent five days in Perth after leaving quarantine, and while infectious. He stayed with a friend in Perth – who returned a positive COVID test on Friday afternoon – and also visited several restaurants and shops across the city.

“We now have two positive cases that have been in our community from 17 April,” Mr McGowan said.

WA health authorities are finalising locations of concern and narrowing down close contacts for the Victorian man and his Perth friend. Her family are also being tested for the virus.

Masks will be mandatory across Western Australia from 6pm (local time) on Friday, while the lockdown in Perth and Peel will begin at midnight.

“I know this is hard to take and I wish we didn’t need to do this. We can’t take any chances with the virus,” Mr McGowan said.

Residents will be confined to their homes for at least three days, except for essential shopping and medical treatment, and an hour’s outside exercise each day.

Crowds at public sporting events and Anzac Day ceremonies have also been cancelled.

“We have gone more than 12 months of no committee transmission and our lives in Western Australia have been normal compared to what we have seen around the world,” Mr McGowan said.

“We do everything we can to protect our way of life.”

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The Victorian man was one of 257 passengers on board Qantas flight QF778 from Perth to Melbourne on April 21. All have been forced into isolation after confirmation of his positive test.

The man, who is asymptomatic, arrived home in Melbourne on Wednesday after completing 14 days of quarantine at the hotel.

He was tested after returning to his home with his wife and two children in the city’s eastern suburbs.

His positive result ended Victoria’s eight-week run of zero local transmission.

Genomic testing has confirmed the virus spread in the corridors of the hotel from a couple who had returned from India.

A pregnant mother and her four-year-old daughter who stayed at the same the Mercure in Perth’s CBD also caught the virus while in quarantine.

The Mercure and two other Perth hotels have been identified as having the same “high-risk” status for ventilation issues.

On Thursday, Mr McGowan said Perth’s Mercure Hotel would not take any more returned overseas travellers after the outbreak.

It will instead transition to a “low-risk” quarantine hotel for seasonal workers expected to arrive from Tonga and Vanuatu in May.

Documents released by the government have revealed the extent to which it was aware of risk factors at its quarantine hotels.

Chief health officer Andy Robertson received a report on April 8 that identified three hotels – the Mercure, the Sheraton Four Points and Novotel Langley – as being “high risk” for ventilation issues.

Dr Robertson wrote to Mr McGowan a week ago, advising that the Mercure was the highest-risk of the three hotels and it should no longer accommodate returned travellers.

The mother, who is six months’ pregnant, and her daughter at the Mercure returned positive tests that day.

In his letter, Dr Robertson said the risks could be mitigated by changes such as installing HEPA air filters in rooms with positive cases.

“Assessment of the three higher risk hotels indicates that the Mercure Hotel is probably the most difficult to mitigate, given positive pressure rooms, opening windows and the age of the facility,” he wrote.

“The other two higher risk hotels (Four Points Sheraton, Novotel Langley) are easier to mitigate utilising measures already implemented and the recommended measures.”

The ventilation report was commissioned after a security guard at the Sheraton contracted COVID-19 in January, prompting a five-day lockdown.

-with AAP