The aircraft had been chartered to transport the passengers to Cooranbong for the day. The pilot had not previously operated into the strip and prior to DEPARTURE he enquired of another operator as to the suitability of the strip for his operation. He determined that the strip was suitable. On arrival in the circuit area the pilot assessed that the wind direction was swinging between 170 and 210 degrees and the velocity was gusting between 10 and 20 knots. The pilot elected to land the aircraft on runway 17. The pilot reported that when the aircraft was on short final it was affected by windshear. The aircraft touched down heavily, short of the runway sealed surface, continued along the ground for 7 metres before the mainwheels struck a low earth bank. Both propellers struck the ground momentarily as the aircraft continued onto the runway. The strip is located in hilly, tree covered terrain and at times is adversely affected by windshear. Two local pilots reported that at about the time of the accident adverse wind conditions did exist.