A crew member on board a ship docked at Kwinana has tested positive to COVID-19.
The man, aged in his 20s, presented to Fiona Stanley Hospital for an unrelated illness on Sunday and had no COVID-19 symptoms.
"He produced a weak test and was sent back to the ship in isolation whilst further testing was undertaken," Premier Mark McGowan said.
"He was then found to be positive and he's been taken off the ship and put into (hotel) quarantine."
Mr McGowan said the man wore PPE while being transported to the hospital and to the quarantine hotel and health staff followed infection control processes.
"The risk to Western Australia is very low," he said.
There are 22 crew members on the Allegra vessel which was due to leave port at 10pm last night.
Mr McGowan said there is a strong chance more crew members could test positive and he wants the Allegra to leave WA as soon as possible.
The State Government are working with Federal authorities to manage the departure of the Allegra from the Kwinana Bulk Jetty.