The pilot was spraying a pea crop in calm conditions. A high voltage powerline was strung above a fence bounding one side of the paddock being treated. The pilot knew of its presence and could see it clearly during the spray runs. At the end of the sixth run prior to making the procedure turn, the pilot felt a slight jolt and was surprised when he saw the powerline collapse. The aircraft's deflector wire contacted the powerline and guided it over the fuselage until it was severed by the deflector wire rear attachment bracket on top of the fin. The pilot advised that he misjudged the clearance between the fence and the powerline and later inspection revealed that the line was hung lower than is usually the case.