The aircraft was engaged in aerial mustering operations approximately 6 km north-west of Napier Downs homestead when the engine suddenly lost power. At that time the aircraft was approximately 150 feet above ground level and heading south at 50 knots. The pilot immediately commenced a descent through tree tops into a small clearing. Just prior to touching down the aircraft was turned through approximately 90 degrees to face into the light easterly wind to avoid overshooting. The pilot stated that the landing was firm, but not heavy. However the tail boom was severed by the main rotor blades and carried anti-clockwise until it struck the main rotor mast head and the roof of the cockpit bubble. It is possible that the tree strikes on the approach may have damaged the tail boom and the firm landing caused the main rotor blades to strike the damaged tail. Subsequent investigation revealed that the fuel shut off valve was not fully open, although it appeared to be. The fault in the fuel shut off valve was due to excessive wear of the ball valve. It was also revealed that the inlet valve for the number two cylinder was bent. Either or both of these defects could account for the loss of engine power. This accident was not the subject of an on-scene investigation.