Two returned travellers have contracted COVID-19 in hotel quarantine in Perth, sparking an investigation into the source of the infections.
The WA Health Department had previously reported the infections as being acquired overseas.
But genome sequencing has since confirmed transmission occurred at the Mercure Hotel Perth.
"The virus was transmitted in hotel quarantine at the Mercure Hotel Perth, as two sets of guests, in rooms opposite each other, had the same sequence of virus - despite arriving from different countries at different times," it said in a statement on Wednesday night.
Other guests who stayed on the same sixth floor at the same time were previously released from the hotel after testing negative.
"These guests will be re-tested and directed to self-isolate until cleared by the public health team," the department said.
People who were in rooms immediately adjacent to the infected people will be asked to self-quarantine for 14 days.
WA's acting chief health officer has begun an investigation into how the transmission occurred.
Members of the public are seen wearing masks in Yagan Square, Perth. Source: AAP
"While similar transmission in hotels in other jurisdictions has not resulted in community transmission, the community must always remain vigilant," the department said.
"The threat of COVID-19 remains real."
Meanwhile, NSW health authorities are on the lookout for more local COVID-19 cases after the virus jumped across rooms in a Sydney hotel quarantine facility.
NSW Health said in a statement on Wednesday that three returned travellers have come down with the same South African COVID-19 variant strain, indicating a quarantine breach may have occurred.
Two of the three were family members and stayed in connecting rooms on the 10th floor of the Mercure Hotel on George St in Sydney's CBD, while the third person was staying in another adjacent room.
The three people arrived on the same flight on 3 April.
NSW Health said it was contacting returned travellers who had been staying on the same floor of the hotel between 7 April and 12 April.
Those people will be directed to get tested and self isolate until 14 days after they left quarantine at the Mercure Hotel, as will relevant hotel staff.
The Mecure Hotel on George St in Sydney. Source: ICE Portal
Health authorities also said that a separate case of COVID-19 transmission between hotel rooms of the Adina Apartment Hotel at Town Hall was ongoing, and all contacts located so far have tested negative.
More than 180,000 virus jabs have been administered to date by NSW Health.
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