The pilot and his passenger were engaged in a pig culling exercise. The weather conditions at the time were very hot and humid with light winds. The pilot was operating at low forward speed along a creek and just above tree top level, when he felt the helicopter begin to sink. He applied full power and turned right but could not prevent the helicopter from colliding with trees. There were no reported mechanical defects which may have contributed to this accident. It is probable the helicopter was being operated on the edge of its performance envelope and there was insufficient power available to maintain level flight when a down draught was encountered.