Police investigating the discovery of a human head washed up on a Rottnest Island beach are using analysis of sea currents and the national missing person's database to search for answers.

A holidaying family made the discovery inside a plastic bag at Porpoise Bay on Sunday.

''They were, obviously, very shocked and we've put in place what we can to assist them with the feelings that they are going through,'' Detective Senior Sergeant Cameron Western of the West Australian Major Crime Squad said.

The head belonged to a Caucasian man about 30 to 50, police believe. It had no front teeth. An examination showed the teeth were lost sometime before their owner met ''whatever fate he's met'', Senior Sergeant Western said.

Police plan to search the rest of the island's southern coastline as part of the investigation.

''We are conducting investigations to determine the currents surrounding that area,'' Senior Sergeant Western said. ''The avenues that things could have arrived from are limitless.''