Twenty-five crew from the Al Messilah livestock carrier at Fremantle Port have tested positive and there are fears others among the remaining 27 crew may also be infected.

One crew member is in hotel quarantine and the rest remain aboard.

One person has also tested positive aboard the Key Integrity bulk carrier which has arrived in Fremantle from Geraldton.

Test results for the remaining 19 crew are expected to be confirmed on Tuesday.

There have now been six COVID-19 outbreaks on ships arriving into WA, including four in the last month.

"It is becoming clear that ships arriving with COVID-19 on board is one of the weakest links and the biggest risk to our way of life in Western Australia," Mr McGowan said.

"We have about 30 vessels arriving at WA ports each and every day which goes to show just how significant the risk is to our state.

"We need a coordinated international approach to this and we need our federal government to take international action."

WA's Department of Health has previously managed outbreaks aboard the Artania, Al-Kuwait, Patricia Oldendorff and Vega Dream vessels.

Authorities plan to take as many crew as possible off the Al Messilah and into hotel quarantine so the vessel can be deep-cleaned.

Tuesday's official case tally will include the latest 24 cases confirmed aboard the Al Messilah.

The premier has also pleaded with the federal government to slow down on allowing international travellers, after 23 people entered Perth from New Zealand via Sydney.

They are all in quarantine.

New Zealanders entering via Sydney or Darwin will be sent to hotel quarantine at their own expense and will be included in WA's cap on international arrivals.

Anyone who does not meet the exemption criteria will be refused entry.

Australian Associated Press