The aircraft was engaged in spraying herbicide. The particular paddock being sprayed consisted of several north/south oriented strips. Earlier in the day the pilot had sprayed the strips in the western section of the paddock. A powerline ran approximately east/west across the northern boundary of the area being treated. The spray runs were aligned south to north, with the aircraft pulling over the powerlines at the end of the northward runs. The pilot then departed to spray another area, with the intention of returning later to complete the small eastern section of the paddock. On return to the area about an hour and a half later, the aircraft was observed to make a spray run from south to north. At the end of this run, the aircraft collided with the powerline, and dived to the ground some 180 metres further north. The aircraft had struck the powerline at a height of about ten metres above ground. Only one of the two conductors was broken. The aircraft had struck the cable with the outboard right wing and tail. Marks on the severed section of the right wing indicated that the aircraft was in an estimated 40 degree left bank at the time. The top of the fin and rudder were also torn off, and the fin and rudder were bent to the right, jamming the rudder.