Forensic experts fear a missing German yachtsman was "hacked to pieces and burned" by cannibals while visiting a remote island in French Polynesia on the trip of a lifetime.

Ash remains, believed to be those of 40-year-old Hamburg man Stefan Ramin, have been found in a valley in Nuku Hiva, the largest of the Marquesas Islands in French Polynesia.

Radio New Zealand International reports tests will be undertaken on the ash, which police found at a fire site after they went to search for Mr Ramin and local guide Henri Haiti.

The tour guide is believed to have taken Mr Ramin on a hunting trip last weekend before he allegedly lured his wife into a remote area and tried to sexually assault her.

Mrs Ramin told police the tour guide tied her to a tree, but she escaped and notified authorities.

Police fear Mr Ramin, from Hamburg, could have been attacked by cannibals and then eaten.

The German couple had been living on a yacht and planned to spend the next few months in French Polynesian waters, Radio New Zealand International reports.