Spraying runs were being conducted over a paddock which had power lines along one boundary. The pilot had been passing beneath the lines during each run, however after completing about two thirds of the task the wire deflector on the aircraft snagged and broke the powerline. The pilot carried out a precautionary landing and discovered that the rudder of the aircraft had been substantially damaged by the wire strike. At the point where the wire strike occurred there was less clearance between the wires and the ground than that available during previous swath runs. The pilot was aware of the situation, but had been subject to a visual illusion, which had led him to believe that there was sufficient clearance to allow the aircraft to pass beneath the wires. By the time he realised that the clearance was insufficient he was unable to take avoiding action, and had elected to allow the wire to strike the deflector, rather than risk the landing gear contacting the ground. The anti-snag deflector plate on top of the rudder had failed, allowing the wire to contact the rudder. The upper portion of this component had then been torn from the aircraft.