Metropolitan Perth and the Peel region have begun
The snap lockdown, which is scheduled to end at midnight on Monday, comes after two people spent several days in the community while infectious.
They include a 54-year-old man who stayed in the Mercure Hotel, the source of the outbreak, and completed quarantine on 17 April after arriving from China.
He spent his first night out of quarantine at the home of a friend, a Kardinya mother-of-two who has also tested positive. Her children have returned negative tests.
The man visited locations throughout the city, including popular tourist spot Kings Park, before leaving Perth on 21 April.
Anzac Day dawn services will be cancelled and people must wear masks when leaving their homes.
Premier Mark McGowan said he hoped the lockdown would not need to be extended.
"I know this is hard to take and I wish we didn't need to be doing this," he said on Friday.
"But we can't take any chances with the virus."
Genomic testing has confirmed the virus initially spread in the corridors of the Mercure Hotel from a couple who had returned from India.
A pregnant mother and her four-year-old daughter who were staying across the corridor tested positive and remain in quarantine at the hotel.
The man who tested positive in Victoria had been staying in an adjacent room.
People can only leave their homes for four reasons: work if they are unable to do so from home, shopping for essential items, medical or healthcare needs or exercising for one hour per day.
Crowds were permitted at Friday night's NBL and Super Rugby fixtures in Perth but had to wear masks.
No crowds will be allowed at Saturday night's AFL clash between Fremantle and North Melbourne at Optus Stadium.
Western Force fans in masks arrive for the Super Rugby game between Western Force and the Queensland Reds at HBF stadium in Perth, Friday, 23 April, 2021. Source: AAP
The man who tested positive after leaving the Mercure flew to Melbourne on flight QF778 which departed on Wednesday carrying 257 passengers.
States and territories respond to outbreak
Victorian authorities are contact-tracing passengers and NSW Health has commenced screening of recent flights from Perth.
Meanwhile, Queensland will require anyone who has been in Perth or Peel since 17 April to comply with the requirements of the WA lockdown.
Tasmania has declared Perth and Peel high-risk regions, with travel and quarantine rules imposed for those coming into the state who may have been in those regions.
The NT has also declared Perth and the Peel region COVID-19 hotspots.
Mr McGowan will seek to limit WA's international arrivals at 512 a week for the next month.
Documents released by the government this week identified the Mercure as one of three hotels considered "high risk" for ventilation issues.
WA's chief health officer received the report on 9 April and wrote to the premier last Friday advising that the Mercure should stop being used.
A general view of the Mercure Hotel Perth in Perth, Friday, 23 April, 2021. Source: AAP
The mother, who is six months' pregnant, and her daughter at the Mercure returned positive tests that day.
The Mercure will soon no longer accommodate returned travellers and the government will review the continued use of the other high-risk hotels.
