Three people who tested positive for COVID-19 while in hotel quarantine in Perth have now been found to have the more transmissible UK variant of the virus.
Premier Mark McGowan said genome sequencing had confirmed the UK strain, emphasising the importance of the state's secure quarantine arrangements.
He said anyone who interacts with the three travellers would need to follow all protocols in relation to dealing with confirmed cases.
He said those precautions remained strong, with WA's quarantine system successfully handling about 30,000 arrivals over the past nine months.
"It just goes to show that we're not out of the woods with COVID," the premier said on Tuesday.
"It's insidious. It's out there, it's causing havoc around the world. It's causing mass deaths in Britain and the United States.
"We are COVID free, and therefore we are very fortunate and I want to keep it that way."
NSW, Victoria and South Australia had all previously recorded cases of the UK variant in returned overseas travellers.
It is believed to be up to 70 per cent more infectious, causing a sharp rise in cases in England and prompting British Prime Minister Boris Johnson to order a new national lockdown until at least mid-February.
Mr McGowan has also backed hotel quarantine for all international flight crew, something introduced in WA six months ago, and the testing of returning travellers before they board flights to Australia, especially those coming out of the UK and the US.
"I think it would be a great safety mechanism to ensure that people coming out of many countries around the world have a test and confirm they are negative before they board a flight with hundreds of other Australians," he said.
"And I think that safety measure should be implemented. To me, this is a no brainer.
"If we do this, we can better protect our citizens."
The premier further took aim at NSW authorities, both over their attitude to the WA's border closures and the decision to allow crowds at the upcoming Sydney cricket Test.
"What you find in New South Wales is the political class there don't seem to understand there is an Australia, outside of Sydney," Mr McGowan said.
"New South Wales seems to think there's an infallibility in New South Wales, that everything they do is perfect.
"Well, it's not. And there's another Australia out there. I think they need to look in the mirror and wake up to themselves."
WA reported one new COVID-19 case on Tuesday, a man in his 30s who recently returned from overseas and is in hotel quarantine.
Australian Associated Press