Perth and Peel will enter a three-day snap lockdown after a Melbourne man spent five days in the city while potentially infectious with COVID-19.
The friend who he was staying with has now also tested positive.
The 54-year-old man arrived in Perth from Shanghai, China, on April 3 and left hotel quarantine on April 17, after testing negative to COVID-19 on his Day 12 test.
He then stayed with a friend and her family in the Perth suburb of Kardinya and visited various locations including a restaurant, a swimming pool and a university college before flying to Melbourne on April 21.
Premier Mark McGowan said one of the people he had been staying with has today returned a positive COVID-19 test, with health authorities still awaiting results from other family members.
Anzac Day marches and other events across the two regions have been cancelled and residents will only be able to leave their homes for one of four essential reasons.
From 6pm this evening, masks will be mandatory for everyone over the age of 12.
Then from midnight, people are only permitted to leave home for work, essential shopping, healthcare, and exercise and places of worship, pubs, clubs, bars, gyms, cinemas, entertainment venues, libraries and outdoor recreational facilities will be shut down.
Cafes and restaurants will be able to provide takeaway only.
No visitors will be permitted in hospitals or aged care. Weddings and funerals can go ahead, but with a 100-attendee capacity.
Residents living outside the restricted Perth and Peel regions will only be allowed to enter to provide essential services or if they are returning residents.