Metropolitan Perth and two nearby regions will enter a five-day hard lockdown after a hotel quarantine security guard attended more than a dozen venues while infected with COVID-19.
Authorities believe the man in his 20s, who worked at the Sheraton Four Points hotel in Perth's CBD, has likely contracted the highly contagious UK variant of the virus and have urged anyone to get tested if they have symptoms.
From 6pm on Sunday, metropolitan Perth, the Peel region and the South West region, will lock down until 6pm on Friday.
All residents must stay home unless shopping for essentials, attending to medical or healthcare needs, exercising within their neighbourhood or working if unable to do so remotely.
Schools which were due to resume on Monday will remain closed for another week.
WA had gone nearly ten months without a virus case in the WA community before the breach.
"I know for many Western Australians, this is going to come as a shock," Premier Mark McGowan said.
"Western Australians have done so well for so long, but this week it is absolutely crucial that we stay home, maintain physical distancing and personal hygiene, and get tested if you have symptoms.
"This is a very serious situation, and each and every one of us has to do everything we personally can to stop the spread in the community."
Mr McGowan said the Maylands man returned a positive test overnight after last working at the hotel on January 27.
He first experienced symptoms on January 28 and phoned in sick that day.
Police Commissioner Chris Dawson said the man of overseas descent also had a second job as a rideshare driver but had not worked in that role since January 22.
He returned three negative tests between January 15 and 23 and it is believed he was likely first infectious from January 26.
The man had worked two 12-hour shifts on January 26 and 27 on the same floor as a person infected with the highly contagious UK variant of the virus.
Authorities believe he has also contracted the UK variant but genomic testing results won't be known until Tuesday morning.
His three housemates have tested negative but have been placed into hotel quarantine for 14 days. Any other close contacts will also be required to isolate.
WA Health has released a list of 15 venues, mostly in Maylands but also including the Perth Convention Centre and a GP practice in Nedlands, that he attended from January 25 to 30.
Anyone who attended those venues must get tested at a COVID-19 clinic and isolate until their results are available.
Authorities are investigating the breach but say the guard did not enter a hotel room.
Mr McGowan said it was unclear whether the man had done the wrong thing by not isolating when he became symptomatic.
"He went to a doctor because he was unwell," he said.
"Sometimes some of the symptoms are not what you might think ... he may not have thought it was COVID."
Chief Health Officer Andy Robertson was unable to judge whether there had likely already been community spread.
"We do get super-spreaders within COVID-19," he said.
"But we've also had others, like the 20-year-old lady in Queensland, who was out in the community for three or four days and didn't manage to spread it to anybody.
"It will vary from person to person."
Face masks were handed out to media as they awaited the premier's press conference.
Earlier on Sunday, both Labor and the Liberals cancelled planned events. Both parties will suspend their election campaigns during the lockdown.
Long queues had already started to form at Perth supermarkets when word spread that a significant announcement would be made.
Mr McGowan urged the community not to panic-buy groceries, saying the state was well equipped to ensure supplies.
Prime Minister Scott Morrison backed WA to crush the spread of the virus.
"The coming days will be difficult for many Western Australians but we're confident WA will be able to get on top of the current outbreak," he posted on Twitter.
"A big thank you to everyone in WA for your patience and cooperation as we all work to fight this terrible virus."