While returning to refuel at a mustering yard, the pilot attempted to move a bull from some trees. When this proved unsuccessful the pilot climbed to continue the flight to the yard. The engine began to run roughly and an approach to a clearing was made. Rotor rpm decayed as some trees were cleared and the pilot was unable to prevent a heavy landing. The helicopter bounced about two metres, the right skid collapsed and the main rotor struck the ground. Investigation revealed that the engine failed due to fuel exhaustion. The pilot's technique was to work on a set endurance from full tanks and not rely on the fuel gauge. At the previous refuelling he filled the tank to the filler neck, but the helicopter was parked on a slope which prevented approximately 20 percent of the tank's volume from being used, and reduced the endurance.