Perth and Peel regions have been plunged into a three-day lockdown after a man spent five days in the community while possibly infectious.
The man contracted COVID-19 while quarantining at the Mecure Hotel in Perth.
He then flew to Melbourne where he tested positive to the UK strain of the virus.
Premier Mark McGowan said the men spent five days in Perth and visited several places.
"After leaving hotel quarantine on the 17th, he stayed with a friend and her two children at their home in Kardinya and went to a Malaysian restaurant," he said.
"That friend has – who is a close contact – had a rapid COVID test today and that has just returned positive."
The Victorian man, aged in his 50s, was also in the Leeming, Northbridge and Kings Park areas and stayed at St Catherine’s College at UWA – which offers short-stay accommodation.
He then boarded flight QF 778 to Melbourne at 1.05pm on April 21.
"That flight had 257 passengers and Victorian Health is working on contact tracing those people," Mr McGowan said.
The three-day lockdown starts from midnight Friday until midnight Monday and masks will be mandatory from 6pm.
All ANZAC Day services in the Perth and Peel regions have been cancelled.
People are only permitted to leave their homes for work, shopping for essential items, medical and healthcare needs and exercise.
Weddings and funerals can proceed but are capped at 100 guests and masks must be worn.
Below is a list of public exposure sites: