As the helicopter was being hovered the pilots heard a muffled bang. The instructor immediately assumed control and landed the helicopter. An inspection of the transmission area revealed that the rear drive belt was missing. The instructor reboarded the helicopter and commenced to hover taxi back to the hangar. Another bang and other noises were heard emanating from the rear of the helicopter, which was landed and the engine shut down. Substantial damage had been caused to the transmission area. The reason for the initial belt failure was not determine as the belt was not recovered. The remaining belts failed after being overloaded during taxi back to the hangar. It is considered that after the belts failed the actuator continued to run winding itself apart. This resulted in the gross misalignment of the driveshaft and in turn the failure of the flexplate and the damage to the transmission area.