The pilot was conducting agricultural spraying operations on a paddock which had a powerline crossing it at an angle and a fence along the western boundary. On completion of a spray run to the north, the pilot returned for a run to the south along the fence line. The marker was required to climb over the fence into the next paddock and move away from the flight path of the aircraft, prior to the commencement of the run along the fence. As he approached the powerline, with the right wing almost over the fence, the pilot noticed that the marker had delayed climbing over the fence. He took evasive action to avoid striking the marker and during this attempt the aircraft contacted the powerline. The aircraft deflector cable failed, allowing the power line to sever the rudder from the aircraft. The pilot carried out a precautionary landing on a road. During the landing roll, the left brake failed and the aircraft ground looped. The investigation revealed that the pilot misjudged the clearance between the aircraft and the powerline. The deflector cable was severed by the impact with the powerline and the rudder was torn from the aircraft. The left main landing gear leg was found to have failed from a combination of bending and twisting. It is considered likely that the brake failure occurred as the leg and attached brake line were torn from the aircraft, depriving the pilot of directional control. There were no pre-existing defects with the aircraft which could have contributed to the accident.