The pilot had been spraying the paddock all morning and, for nearly four hours, had been crossing a set of wires running across the field almost at right angles to the aircraft's flight path. On the third run of the fifth load for the day the aircraft's main wheels contacted the top wire of a two wire set, and the forward motion was arrested causing the aircraft to impact the ground. During the ground impact sequence the pilot was thrown from the aircraft when a seat belt anchor point failed due to corrosion. The pilot had no recollection of any events during the day of the accident. Determination of the actual position of the wire was possibly made more difficult by the wire and one of the support poles tending to blend into the background. A possible contributory factor was that the pilot's concentration may have been reduced due the death of a relative the day before the accident.