The Piper Arrow was being flown by a pilot undergoing a Commercial pre-licence check, under the supervision of a Grade 1 instructor. At a height of 3450 ft, some 2 NM west of the Wedderburn ALA, the instructor closed the throttle to simulate an engine failure. The weather was fine, with a light wind of about 10-15 kts from the west-south-west. The instructor reported that the pilot elected to conduct a glide approach to the ALA, joining the circuit on early downwind for a landing on runway 17. All required checks were correctly carried out, including exercising the throttle to prevent excessive engine cooling. An initial aiming point of about 1/3 of the strip length in from the landing threshold was used. The landing gear was lowered on base leg. Drag was progressively increased by extending flap and moving the propeller pitch to fine, during base and final legs, in order to bring the aiming point back to about 100 m beyond the threshold. At about treetop level, on short final, an area of sink was encountered which resulted in the aircraft developing a high rate of descent. Although both the pilot and instructor selected full throttle, they were unable to prevent the aircraft from touching down heavily on the runway threshold. The aircraft came to rest after a 30-40 m landing roll. The nose landing gear had been dislodged, the main landing gears had been bent rearwards, and substantial buckling of the wing was evident. Both occupants were uninjured and were able to vacate the aircraft normally.