Three people who tested positive for COVID-19 while in hotel quarantine Perth have now been found to have the more transmissible UK variant of the virus.
Premier Mark McGowan said the genomic testing had confirmed the UK strain, emphasising the importance of the secure quarantine arrangements.
"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.
"I want us to keep the virus out and make sure that we do everything we can to prevent it from coming into our state, including keeping borders in place."
Mr McGowan has also backed testing returning travellers before they board flights to Australia, especially those coming out of places like the UK and the USA.
"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."
