A fly-in, fly-out mine worker has tested positive to coronavirus in Western Australia and may have been infectious while in the community.
Premier Mark McGowan says the man returned a "very weak" positive test result at his workplace in recent days but has since tested negative.
He had previously tested positive to the virus in March or April last year.
The man worked at Fortescue Metal Group's Cloudbreak mine in the Pilbara between 20 July and 27 July and subsequently attended venues in Fremantle, Scarborough and Subiaco.
He is believed to have contracted the virus at Perth Airport from a Queensland man who flew into Perth last month and was subsequently denied entry to WA. That man tested positive to the Alpha variant of the virus upon returning to Brisbane.
"The best explanation we have for his weak positive test result is that he probably acquired it from the man at the airport through some sort of casual contact," the premier told reporters.
"We think he was positive over the course of the last 16 days ... (but) there's a strong prospect he wasn't infectious because he was COVID-positive early last year."
Health Minister Roger Cook said WA Health would continue to update a list of six exposure sites as new information emerged. The sites .
Anyone who attended those sites must get tested immediately and quarantine until they return a negative result.
"There will be a large pool of casual contacts," Mr Cook said.
"There's no reason to believe this will lead to a lockdown but there's very much cause for concern."
The FIFO worker's girlfriend has tested negative, as has one of his three closest workmates. Results are pending for two other co-workers and a flatmate.
All are being treated as close contacts and must quarantine for 14 days.
Several ships have arrived in WA with infected crew members after making stops in coronavirus-plagued Indonesia.
They include two ships currently docked in Fremantle, the BBC California and Darya Krishna, both riddled with the highly-infectious Delta strain of the virus.
Two crew members from the Darya Krishna are in hospital, with one in intensive care. A further five infected seafarers remain on board.
The BBC California is expected to depart in the coming days once the last remaining infected crew member is given the all-clear.
Mr McGowan has threatened to turn away ships suspected of carrying infected crew and to demand evidence of negative test results before vessels enter WA waters.
But Ports Australia chief executive Mike Gallacher said WA needed to work with the maritime industry to keep trade alive.
"Ports Australia is not aware of one instance where a COVID-19 case onboard a vessel pulling into an Australian port has resulted in community transmission, which simply means our protocols are working," he said on Wednesday.
"Yes, we need a practical approach to safeguarding our people and economy, but we also need a humanitarian approach to protecting international seafarers who make trade possible."
