After an aero tow to 2000 feet above ground level, the two seat Grob glider descended before the pilot located a thermal. The flight was to give a visiting Japanese glider pilot an experience of gliding in Australia. Two other gliders were launched and they commenced thermalling in an adjacent thermal. The Grob pilot later joined the other gliders in their thermal, and remained there while climbing from 2500 feet to 3000 feet above ground level. With the other gliders still in the thermal and at least 200 feet higher, the pilot elected to leave and tracked north at 50 knots. A short time later the glider experienced a collision and the pilot observed another glider immediately above and overtaking. Suspecting tail damage, the pilot elected to conduct a descent at low speed without the use of dive brakes. The descent in sinking air took about 20 minutes. Both aircraft landed uneventfully. The pilots later concluded that the single seat glider had departed from the thermal shortly after the two seater, without the pilot of the single seat aircraft being aware that the other had left. They both departed in the same direction. Because the inter-thermal speed for the single seat glider was 70 knots the pilot accelerated to that speed, using the 200 feet of vertical separation that had existed with the two seat glider. The result was that neither pilot was able to see the other aircraft prior to the collision.