Premier Mark McGowan said it is a positive start after 3171 people got tested on Sunday and 13 close contacts returned negative test results.

But he urged people to continue getting tested and said public metropolitan clinics will remain open until 10pm each day.

"We need to get more tests done," he told reporters on Monday.

"These are encouraging signs but it's still early days."

It comes after a security guard at the Sheraton Four Points hotel in Perth's CBD contracted the highly-contagious UK variant of the virus, then attended more than a dozen venues over several days while infectious.

Genomic testing has confirmed the man contracted the UK strain.

The premier on Monday announced WA Police will launch a full investigation into how the quarantine breach happened.

Former WA chief health officer Tarun Weeramanthri will also undertake another review into the adequacy of WA's hotel quarantine system.

"We will leave no stone unturned to find out exactly what happened to prevent it from happening again," Mr McGowan said.

Authorities have identified 66 close contacts of the man who has been dubbed "case 903". They have been contacted and told to self-isolate.

Eleven "high-risk" contacts, including the man's three housemates, have been placed into hotel quarantine as a precaution.

The security guard, aged in his 20s, is also a rideshare driver but authorities believe he had not worked in that job since January 22 - several days before likely becoming infected.

Metropolitan Perth, the Peel region and South West have completed their first night of a five-day lockdown which will run until 6pm on Friday.

All residents must stay at 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.

Australian Medical Association WA president Andrew Miller has labelled the breach predictable, saying the McGowan government had ignored concerns about the hotel regime.

"It's incredibly disappointing that we are still running what we would describe as an amateurish quarantine system," he said.

Dr Miller called on WA to invest in dedicated quarantine facilities not used for any other reasons, proper airborne protection including fresh air ventilated through hotels and supply of N95 face masks for all security guards.

He also wants guards to be better paid and banned from taking a second job.

Authorities are pleading with West Australians not to panic-buy after chaotic scenes at supermarkets and pharmacies.

People must wear face masks anytime they leave the house but with many pharmacies sold out on Sunday, the police commissioner has encouraged people to tie a scarf or bandana around their face while going out to buy one.

WA Health has released a list of 16 venues, mostly in Maylands but also including the Perth Convention Centre and a GP practice in Nedlands, that the guard visited between January 25 and January 30.

Anyone who attended those venues must get tested at a COVID-19 clinic and isolate until their results are available.

One new venue was added to the list on Monday - the Grab N Go convenience store on Milligan St in Perth.

The man returned a positive test on Saturday night after last working at the hotel on January 27. He first experienced symptoms the following day.

A man who posted on social media about having tested positive while at Fiona Stanley Hospital is not believed to be a threat to the community.

WA Health said it appeared to be a historic case which was infectious overseas.

All other states have moved to either close borders or restrict travel from WA.

Australian Associated Press