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A former police forensic superintendent believes the ocean dumping of a human head found washed up in a plastic bag at Rottnest Island will complicate the investigation.

Adjunct Associate Professor Hadyn Green, who is teaching forensics at the University of WA, said the environment and insect activity provided the best information about the time of death.

Police and State Emergency Service volunteers yesterday wound up their search of the beach and rocks at Porpoise Bay, on the south-east of the island.

It was reported that police know how the man died.

Professor Green said that some flies were attracted to a body within minutes and if experts could tell when the insect activity started, it could help police determine when the man died.

But he said it was hard to know what had happened to the man's body after he was killed, whether it was frozen for a time and then exposed to the open air before being put in a bag and dumped in the sea.

Bodies decomposed slower in water than the open air because of the cooler temperatures and if the head was completely sealed in the plastic bags, that could also slow decomposition.

Professor Green said it was likely to be a long investigation.

The murder investigation was sparked when a family holidaying on Rottnest made the gruesome discovery of the head inside two plastic bags on the shore at Porpoise Bay on Sunday afternoon.

Major crime squad detectives trying to identify the murdered man say they have had several calls from the public providing them with "avenues of investigation" and have asked anyone with information to come forward.

Further forensic testing, including for DNA, is being done to try to identify the man. But if he had not provided a DNA sample during a criminal investigation or through his employment, police may not find a match on their database.

Anyone with information should call Crime Stoppers on 1800 333 000.