The aircraft was engaged in aerial spraying operations. The weather at the time of the accident was reported as fine, with clear skies and calm wind conditions. Toward the end of a swathe run, involving a descent into a small valley followed by a climb over rising terrain, the pilot was required to manoeuvre the aircraft over a powerline. The pilot reported that, as the aircraft approached the powerline, the engine experienced a partial loss of power and did not respond when full power was applied. He elected to fly beneath the powerline, in order to avoid it, but considered this action may have been taken too late as he was unable to arrest the aircraft's rate of descent in time to negotiate the rising terrain on the far side of the powerline. The contents of the hopper remained on board the aircraft during the accident sequence as the pilot did not have an opportunity to jettison the load. The aircraft impacted heavily onto rocky ground about 50 m beyond the powerline and was substantially damaged. The pilot escaped without injury. As the Bureau did not conduct an on-site examination of this occurrence, the reason for the apparent loss of power could not be determined.