Three of Perth's quarantine hotels are deemed "high-risk" for ventilation issues but only one will be closed after becoming the source of a COVID-19 outbreak.

A COVID-19 outbreak in a Perth quarantine hotel has spread to Melbourne as it emerged two other hotels have the same "high-risk" status for ventilation issues.

A third case has been linked to the Mercure Hotel after a man in Victoria tested positive on Friday morning.

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

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

The man was staying in the room adjacent to the couple.

A pregnant mother and her four-year-old daughter who were staying across the corridor have also tested positive and remain in quarantine at the hotel.

Premier Mark McGowan has announced the Mercure will no longer accommodate returned overseas travellers.

It will instead transition to a "low-risk" quarantine hotel for a flight-load of seasonal workers expected to arrive from Tonga and Vanuatu next month.

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 which identified three hotels - the Mercure, the Sheraton Four Points and Novotel Langley - as being "high risk" for ventilation issues.

Dr Robertson wrote to the premier last Friday 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 to the premier, 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.

Australian Medical Association WA president Andrew Miller said it was unacceptable that previously healthy travellers had contracted the virus within hotel quarantine, something which has also happened in other states.

Mr McGowan said WA's nine quarantine hotels were accommodating more than 2000 international arrivals on any given day.

He said the federal government's refusal to consider his proposal to shift returned travellers into Commonwealth facilities such as military bases or detention centres had left states running "imperfect" quarantine hotels.

Guests who had stayed on the same floor at the Mercure have been re-tested and directed to self-isolate until cleared.

The floor has been cleared of guests.