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"We cannot underestimate this virus. We cannot be complacent. We need to follow the health advice. We need people to get tested. And we need everyone to keep wearing your masks," McGowan told reporters.
"I want to avoid going into lockdown again. I know how much it can impact people's lives and businesses but if we need to go back into lockdown we will.
"That decision will be based on the latest health advice but currently, at this point in time, we can avoid a lockdown, purely based on the restrictions in place over the past weeks and the requirement people have to wear masks. It has significantly reduced the risk of transmission."
To "protect the West Australian community", the Premier said they'd made the decision not to permit spectators at the AFL Derby at Optus Stadium this afternoon.
Nightclubs will also be closed immediately in WA "in addition to the no-crowd decision".
"The Ministry of Health carried out a rapid public health assessment on the impact for New Zealand last night and, after working with airlines, all direct passenger flights from Perth to New Zealand were paused," the ministry said earlier today.
Asked if WA could "still be headed" for a lockdown this week, Premier McGowan said "it is a prospect".
"But what has saved us from having a lockdown immediately is during the course of this week, last week, when these people were in the community, we were in a state of semi-lockdown, everyone was wearing masks, and we had a very quiet society," he said.
"And that has avoided it at this point in time. Had what happened in the last couple of days occurred three weeks ago, we would have gone into lockdown immediately. But the situation was somewhat different because of the fact we are already in a very precautionary state across Perth and Peel region."
The infected security guard, a man in his twenties, had already had his first dose of the coronavirus vaccine, McGowan said.
He had worked in the Pan Pacific Hotel on April 24, 25 and 26, during which time new arrivals were admitted to the hotel, and worked on the same floor as two returned travellers who had tested positive.
"CCTV vision is being reviewed, however there is no clear explanation at this point as to how the security guard could have been infected," the Premier said.
"It is possible transmission could have occurred while the international arrivals were admitted into hotel quarantine and possibly were transferred to their rooms. This is being investigated further."
The potential for a second lockdown comes not even a week after the Perth and Peel regions were sent into a three-day circuit breaker over the Anzac Day long weekend, following a breach in the WA capital's hotel quarantine system.
McGowan has defended the circuit breaker – brought on by just two cases – saying he didn't want to "see what happened in America or Britain or India or Brazil happen here".
"There's two places in the world, countries of significant size that have avoided that, and that's New Zealand and Australia. And that's basically because we have used our isolation, our borders and our measures using our public health initiatives to good effect and I want that to continue."