FACTUAL INFORMATION The aircraft was involved in aerial spraying operations. The pilot was preparing to do a clean-up run along the edge of a paddock on a heading of approximately 135 degrees. He initially flew due west alongside a powerline. He then turned right to fly under the powerline and return to begin the clean-up run. While approaching the start of the clean-up run, the pilot's attention was focussed on the east-west powerline which he was to again fly under. Before reaching that powerline, the pilot suddenly saw another wire immediately in front of him. He pushed the yoke hard forward to minimise the damage to the aircraft, but the powerline hit the tail area. The aircraft deviated towards the ground before he was able to regain control. The top third of the vertical stabiliser had been removed and the rudder destroyed. Even though directional control was severely affected, the pilot was able to fly the 10 NM back to the airstrip and land without further incident. The pilot reported that the property owner had briefed him on the location of four powerlines in the area, including the powerline he had flown under. He did not brief the pilot on the powerline that was hit. The pilot stated that the property owner appeared to assume that he knew the location of all the relevant powerlines as he had sprayed in that area a few weeks prior to the accident. The pilot reported that the powerline was difficult to see as it was not marked and the weather conditions were overcast. It was a clear span powerline between a post located amongst some trees and a shed. ANALYSIS The pilot was not aware of the location of the powerline which the aircraft impacted at the commencement of a spraying clean-up run. Although the pilot had been informed of four other powerlines in the area, he had not been briefed on, or noticed, this particular powerline. The clear span wire would have been difficult to detect as it was not marked, was observed against an overcast background, and the post was located amongst trees.