The helicopter was on a scenic flight over the Bungle Bungle National Park. As it passed an outcrop it appeared to be rocked by turbulence. The helicopter yawed to the right and commenced an uncommanded descent. The pilot stopped the descent and checked the engine indications. Although no warning lights were illuminated the rotor and N2 RPM were still increasing. The pilot was able to stop the N2 overspeed at 104 per cent. He turned the helicopter towards the airfield and commenced a cruise descent. As the helicopter descended through 150 ft, on final approach, the engine-out alarm sounded and the pilot heard the turbine RPM decreasing. He turned the helicopter into wind and completed an autorotational approach, landing short of the airfield. After landing, the pilot observed the engine was still operating at idle RPM. Post-landing inspection disclosed damage to the tail boom, probably as a result of a heavier-than-normal touchdown. It was reported the rotor and N2 RPM problems were the result of a sheared engine governor drive shaft.