The pilot had been engaged in glider towing operations for about four months, and had completed 108 towing flights. During the afternoon the pilot had carried out two aerotow flights without incident. On the accident flight a normal take-off and transit to the north side of Korumburra township was made. The glider was released at a height of 2000 feet above ground level and the tug aircraft then turned and tracked towards a right base leg position for the south west landing strip at Leongatha. Not all of this flight was observed, but two witnesses noticed the aircraft descending in a spin to the right. It appeared to recover briefly, with the nose being raised above the level flight attitude, however a spin to the left then commenced. This spin continued until the aircraft disappeared from sight, but the wreckage distribution and impact marks indicated that the pilot had been able to stop the rotation in the last moments of the flight. It was evident that insufficient height remained to effect a full recovery. A detailed inspection of the wreckage did not disclose any defect or malfunction with the aircraft, its engine or systems that might have contributed to the development of the accident. It was considered unlikely that the pilot had deliberately entered a spin on his return to the airfield. The aircraft type is not approved for spinning, and the spin characteristics of this particular two seat conversion are unknown. There was no evidence available to determine how or why the spin situation developed. It was apparent that the pilot had succeeded in partially recovering from the initial spin, however the recovery technique being employed did not prevent a spin in the opposite direction.