During a mustering flight, the spotter asked the pilot to drive some cattle towards the stock yards. The helicopter was flying slowly just above the tree canopy, when the spotter realised that something was wrong. Instead of turning to round up the cattle, the helicopter flew straight ahead, descending slowly. The pilot did not say anything but the experienced spotter noticed that he was busy moving the pedals and cyclic control. When the main rotor sliced into the overhanging canopy of a large tree, the spotter jumped from the helicopter. The main rotor stopped when it struck the trunk of the tree. The helicopter then fell to the ground, coming to rest on its right side. The helicopter was being operated at or beyond its Out of Ground Effect (OGE) hover capability. The OGE capability was reduced by an operating weight in excess of the aircraft's maximum permitted weight and a loss of flight performance, caused by an internally mistimed magneto and, possibly, excessive control movements made by the pilot. Although the pilot had received some training in low flying he had not received any training in the problems associated with operations in the mustering environment. The crash was probably survivable had the pilot's helmet remained on his head during the impact sequence. When the helmet was found, the chin strap was not fastened.