The aircraft was engaged in spraying operations. Several days prior to the accident the starter motor had failed. Since then, the engine was started by hand swinging the propellor. On this occasion the pilot attempted a start with the cockpit unoccupied. The engine started and immediately accelerated to a high power setting. The parking brake was unable to prevent the aircraft from moving off and the pilot was unsuccessful in his attempts to board the aircraft. The aircraft completed a 180 degree turn, collided with a motor vehicle and then proceeded across a road where it collided with an earth bank and a tree. The pilot said he may have inadvertently bumped the throttle control to the open position whilst setting the cockpit controls prior to engine start.