The helicopter was engaged in cattle mustering operations. Whilst in level flight at an altitude of 200 feet, the pilot heard a loud noise and felt severe vibration from the engine, which subsequently failed. Following auto-rotation to ground impact, the aircraft rolled onto its side. The semi-conscious pilot was assisted from the wreckage by the passenger who had evacuated the cockpit unassisted. Strip examination revealed that the engine had failed due to broken conrod bolts and a separated conrod big-end cap from the No. 4 cylinder conrod assembly. This had resulted in internal mechanical damage sufficient to stop the engine.