POLICE have concluded the search for human remains on Rottnest Island, where a human head wrapped in a plastic bag was found.

Detectives need help with their investigation into the human remains, which were found about 3pm on Sunday, January 6.

No more remains were found today during a search of the southern side of the island and searchers, including police divers and SES personnel, have returned to the mainland.

Police and SES volunteers finished their search of the beach and rocks at Porpoise Bay as detectives continued to investigate the identity of the man whose head was discovered by a holidaying family.

Divers had been drafted in to search the waters adjacent to the beach this morning, as SES personnel and police scoured the sand.

Porpoise Bay, which had been cordoned off after it was declared a protected forensic area, has now been reopened to the public.

The area was closed after the head, double wrapped in a plastic bag, was found by a family as they combed the beach on Sunday afternoon.

A WA police spokeswoman said there had been no major developments.

"The search in Rottnest has concluded with the majority of SES personnel returning to Perth via ferry this afternoon,'' she said.

Camera Icon Det Sen Sgt Cameron Western of the Major Crime Squad speaks to media about the discovery of a human head in a plastic bag at Rottnest. Credit: PerthNow, Kerris Berrington

HORRIFIC FIND

An 11-year-old girl beachcombing with her family at Porpoise Bay, on the southeast of the island, discovered an item wrapped in a plastic bag just after 4pm on Sunday.

She opened the bag and saw what looked like a human ear, and the family immediately contacted police.

Police confirmed the severed head belonged to a Caucasian male, aged between 30 and 50.

The man's front teeth were missing, with police saying they appear to have been removed some time before his death.

Police are treating the investigation as a murder probe, but would not detail any other wounds on the head or decomposition.

Detectives have been sifting through missing persons files across Australia since the grisly discovery.

Detective Senior Sergeant Cameron Western, of WA's Major Crime Squad, said officers would pursue all avenues to identify the remains, and how they came to be on the beach, 18km off the Perth coast.

"The avenues the remains could have arrived from are limitless,'' Sen Sgt Western said.

"But he appears to be Caucasian and missing his front upper teeth which the pathologist believes is historic and did not occur recently.''

Sen Sgt Western said the family that made the discovery was understandably shocked.

"We have put in place what we can to assist them with the feelings they are going through,'' he said.

Camera Icon Credit: PerthNow

A post mortem is being conducted on the remains, while police look into the shipping movements close to the island, and water currents in the area.

People who were on boats in the bay at the time of the discovery will also be questioned.

Police say anyone with information can call Crime Stoppers anonymously on 1800 333 000.

with AAP

View Body in bag at Rottnest island in a larger map