Test results over the coming days are set to reveal whether Western Australia has dodged a bullet with its COVID-19 outbreak.
No new cases were detected on Monday after a hotel quarantine breach forced metropolitan Perth, the Peel region and South West into a five-day lockdown.
WA's government took the drastic step after a security guard at the Sheraton Four Points hotel in Perth's CBD contracted the highly contagious UK strain, then attended more than a dozen venues over several days while infectious.
More than 3100 people were tested on Sunday, most later in the day after the lockdown was announced.
Authorities expect most of the guard's close contacts will have been re-tested by Wednesday, providing clarity as to whether the lockdown is likely to end as scheduled at 6pm on Friday.
"If they're still coming up as negative, that would be a good sign that this person wasn't a particularly effective spreader," WA's chief health officer Andy Robertson said.
"And that is what we're not quite sure on."
Australian Medical Association WA president Andrew Miller said it was "50-50" as to whether an extension to the lockdown would be needed.
A general view of empty streets looking down towards St Georges Terrace in the Perth CBD during the first morning of the lockdown in Perth on Monday. Source: AAP
"We're hoping for donut days throughout the week - all zeroes - and then perhaps that will be enough," he said.
"But we will know by then whether we're looking at a cluster or clusters which will require a longer period of time, because we know that each cluster usually generates about three weeks worth of cases before it is able to be shut down."
Premier Mark McGowan has welcomed the initial results but urged people to continue braving long queues and hot weather to get tested.
Anyone living in the lockdown zone, including school students, 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.
Mr McGowan said most people had been compliant despite some "disappointing" panic-buying at supermarkets.
It remains unclear how the guard, aged in his 20s, contracted the virus, having assured police he did not enter any rooms.
He is said to be fully cooperating with a police investigation.
Authorities have identified 66 close contacts of the man dubbed "case 903". They have been ordered to self-isolate.
Eleven "high-risk" contacts, including his three housemates, are in hotel quarantine.
Dr Miller has labelled the breach predictable, saying the McGowan government had ignored concerns about the hotel regime.
Police are seen monitoring the Perth CBD amid a five-day lockdown on Monday. Source: AAP
"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.
People in Australia must stay at least 1.5 metres away from others. Check your jurisdiction's restrictions on gathering limits. If you are experiencing cold or flu symptoms, stay home and arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080. News and information is available in 63 languages at .
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