POLICE in Western Australia are investigating the discovery of what appears to be human remains stuffed into a plastic bag that washed up on a beach on Rottnest Island.

The grim discovery was made on late on Sunday afternoon at Porpoise Bay, on the southeast of the island, with police immediately called in to investigate fears the bag contained a body.

Police media spokesman Sergeant Gerry Cassidy told 6PR radio this morning the grim find was made about 4.30pm by a visitor to the island.

"It’s yet to be confirmed as human remains but that is what we fear or suspect it may be,” he said.

"Inquiries will continue today into what appears to be human remains found in a plastic bag washed up at Porpoise Bay.

"It will be examined today by a forensic pathologist and the area of Porpoise Bay at this stage remains a protected forensic area and access to the public is blocked in that general area."

Rottnest Island, situated 18km off the coast of Perth, has been a popular holiday spot for West Australians for generations.