Forensics officers were at Rottnest Island last night examining what is believed to be a human head found in a bag at Porpoise Bay.
It is understood a member of the public made the grisly find early yesterday evening and alerted Rottnest police.
Major crime squad detectives were on the island last night and said a preliminary investigation indicated the remains were human.
"We believe it to be the remains of a human head," Insp. Denise Grant said last night.
"However, we are yet to confirm this. We can't confirm it until the forensic pathologists have finished their examination."
Police would not say what type of bag the remains had been discovered in.
They are believed to have been washed up on the shore and to have been in the water for some time.
More extensive testing will be needed before it is known whether the remains are from an adult or child, and whether they are male or female.
That testing is expected to take place on the mainland today.
Water police and other officers will comb the bays around Porpoise Bay today to try to find other remains.
It is believed a number of bays in the area will be closed while the search continues.
Exactly where the bag was found in Porpoise Bay is unknown.
Water police Sen. Const. Craig Lawson confirmed detectives and forensic officers had been ferried to Rottnest about 8pm.
A visitor to the island said a young girl found the bag late yesterday.
He said she was with her family when she found the double-bagged remains washed up on the Porpoise Bay beach.
The bay is at the south-east corner of Rottnest, several kilometres from the main settlement.
It is sheltered by reefs but debris often accumulates on the beach.