While approaching 6000 feet on descent into Canberra the aircraft entered an area of severe turbulence. The turbulence had not been forecast and lasted for about one minute. Prior to penetrating the turbulence the flight attendants had prepared the aircraft for landing but not all had taken their seats. Two of the non seated flight attendants were thrown about the cabin but were not injured. The aircraft was climbed to 8000 feet for a smoother ride to allow all on board to be seated. One seated flight attendant had the shoulder harness pull through the bulkhead fitting. It was suspected that the harness had been incorrectly routed through the fitting. A fleet check was initiated to inspect all seat harnesses for correct installation. No further examples of incorrect installation were found.