An approved training pilot was conducting mustering endorsement training with another company pilot. At the time of the accident they were practising autorotational approaches with power termination prior to touchdown. The training pilot had demonstrated a number of autorotational approaches from low and medium altitudes. He had also followed the trainee pilot through on the controls for a number of similar approaches. The trainee had exhibited problems with airspeed and rotor rpm control. During the final practice, with the training pilot again following through on the controls, the trainee misjudged the approach. The training pilot decided to terminate the approach at 50 - 70 feet above ground level and with 30 - 35 knots forward speed. Although recovery action was taken the aircraft continued to descend, touching down heavily before the action had any significant effect on the rate of descent. The aircraft slid for approximately 30 feet before overturning. It is possible that the training pilot was distracted by his instructional duties and he misjudged the height required to recover from the autorotational descent.