Concerns are held for up to six Fremantle Port workers who boarded a live export ship before an outbreak of COVID-19 on the vessel was confirmed.
It comes amid growing tension over whether state or federal authorities were responsible for workers boarding the Al Kuwait after news emerged some of the crew had been unwell.
WA Police Commissioner Chris Dawson says a report from the vessel was submitted to the federal Department of Agriculture advising some of the crew had fevers on Wednesday.
It was en route from the United Arab Emirates and the ship arrived in WA two days later.
Six of the 48 multinational crew have since tested positive to the virus.
The federal department says it had been told three of the crew were ill, but none of them had elevated temperatures or COVID-19 symptoms before their arrival.
It only learnt crew members had fevers when they landed and immediately notified the WA health department, it said.
"The WA State Department of Health is responsible for the management of this incident, including all decisions relating to human health," the federal department said in a statement.
"The Department of Agriculture, Water and the Environment is confident that government protocols were met."
WA Premier Mark McGowan says they are being isolated and contact tracing is underway.
Crew members on board the Al Kuwait reported fevers before docking in Fremantle on Friday. Source: Supplied
Mr McGowan said he doesn't want to "point fingers at this point in time" but state authorities are working to find out what went wrong.
"We're very concerned and to a degree, disappointed," he said on Tuesday.
"Clearly, if there's cases of people reporting high temperatures on board ... red flags should be raised.
"So I just say to everyone involved, in particular the Commonwealth agencies that knew about this, they need to be on guard and be alert."
He said the men who tested positive were being quarantined at a Perth hotel while the remaining 42 appeared well and were being kept on board.
They would be monitored and given health assessments as required.
"This is a fast-evolving situation. But I suspect it is probably more than likely that more crew members may become infected with the virus," Mr McGowan said.
Commissioner Dawson says he has been assured none of the crew disembarked.
Mr McGowan said the state government wanted the ship to set sail as soon as possible, but it needed to be cleaned first.
Mr Dawson suggested the federal agriculture department should do the fumigation, saying: "the Commonwealth have responsibilities here."
Federal Agricultural Minister David Littleproud said the ship had notified authorities that there were sick people on board, but they did not have elevated temperatures.
Western Australia Premier Mark McGowan speaks to the media during a press conference in Perth. Source: AAP
Mr Littleproud told Perth radio station 6PR that it was only after the ship had docked that it reported some crew had elevated temperatures.
Mr McGowan said it wasn’t until Sunday evening that Fremantle Port Authority learned of health concerns on board through "word of mouth".
Border Force said it completed all customs and immigration-related clearances, but received no reports of illness on board the vessel on the day of or prior to its arrival in port.
The six new cases bring the number of active cases in WA to 12 after a Victorian family returned from Qatar with the disease on 17 May.
Those four cases were confirmed on Monday, ending a streak of zero case days.
The family has been quarantining in a hotel since arriving.
"This situation again highlights that we are not free from this virus," WA Health Minister Roger Cook said.
"There are still people getting sick and we are still at risk.
"We cannot be careful enough."
The state government remains adamant the interstate border will remain closed for months.
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