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 department can confirm 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 evening.

"All other guests who previously stayed on the same (sixth) floor during this time and have been released from the hotel facility tested negative prior to release. These guests will be re-tested and directed to self-isolate until cleared by the public health team."

The department said all guests who had been staying in the rooms immediately adjacent would be asked to self quarantine for 14 days.

"The acting chief health officer has immediately commenced an investigation into how the transmission occurred," it said.

"While similar transmission in hotels in other jurisdictions has not resulted in community transmission, the community must always remain vigilant. The threat of COVID-19 remains real."

Australian Associated Press