Intensive gliding activities were taking place as part of a course in glider flying. Two tug aircraft were employed in towing operations, and most flights in the gliders did not exceed about 20 minutes duration. The pilot of VH-PXT towed VH-GVZ to about 3000 feet before releasing and returning for the next glider. This aircraft was then towed to 3000 feet and released, following which the pilot of VH-PXT descended to join the traffic circuit on the downwind leg. VH-GVZ had by this time descended and was also on downwind for landing. The pilot of VH-PXT manoeuvred to join in the latter portion of this leg, and witnesses observed the aircraft converging with VH-GVZ from above and behind. The left wing of the aircraft struck the right side of the glider, and about 75 centimetres of the left wing was seen to be hanging vertically as the aircraft entered a spin to the right. This spin continued until impact with the ground occurred. The instructor in the glider was able to retain control of his aircraft, and carried out a safe landing. It was evident that the pilot had not seen the glider until shortly before the impact. Witnesses had observed the aircraft commence a steep turn to the right immediately before the left wing struck the side of the glider. Weather conditions were good at the time, and the reason the pilot had failed to see the glider earlier could not be established. Both aircraft were equipped with suitable radios, but in common with the various other aircraft in the area, were not broadcasting their positions in the circuit.