Western Australian police have released further information about the severed head which was found at Rottnest Island on Sunday.

An 11-year-old girl discovered the head in a plastic bag at Porpoise Bay, but a police search failed to find any more remains.

Police say the victim is a Caucasian male, possibly aged between 30 and 60 years, who had seven front upper teeth missing.

He did not wear a plate or dentures, and the rest of his teeth were in poor condition.

He had a full beard and moustache.

On Monday Senior Sergeant Cameron Western confirmed police are treating the incident as a homicide.

He said police have checked sea currents to find out where the head could have come from.