"Yesterday I undertook another PCR test and the result came back positive," Mr McGowan said in a statement released on Friday morning.

Mr McGowan, who has been in isolation since Wednesday, said the result was "not surprising" given the positive test result of a family member earlier in the week.

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"In accordance with the protocols, this will extend my quarantine period until I am hopefully cleared to leave home next Thursday afternoon, at the earliest," he said.

"I will continue to quarantine and work from home over this period."

The Premier once again thanked testing clinic staff across the State.

"Their ongoing efforts have enabled us to not only detect and monitor the spread of COVID-19 in the community, but also allowed us to keep on top of the evolving challenge we are navigating throughout our State," he said.

"I would also like to acknowledge the vital contribution of our committed health workers who have not only cared for my family, but also the many Western Australians who have been affected by this virus.

"The WA community has been fantastic throughout this global pandemic, and WA's success to date is a credit to each and every Western Australian who continues to follow the health guidelines and keep our community safe, and our economy strong."