The pilot reported that he was spraying thistles on the eastern side of Mt Koang. The weather at the time was clear with a moderate easterly breeze blowing. Spray runs were made in a north south direction, starting at the base of the mount and progressing up the slope parallel to a ridge line at the top. The pilot commenced the last run from the north, flying along the top of the ridge line. Approximately half way along the ridge line the aircraft hit a wind gust. The pilot was unable to prevent the aircraft from hitting the ridge. The aircraft hit the ground heavily and was destroyed in a post impact fire. The pilot was wearing fire resistant overalls, gloves and a helmet. He received burns to his face which was the only exposed area of skin when the accident occurred.