The aircraft was engaged in dropping parachutists. Prior to commencing the flight the pilot reported that he inspected the airstrip and calculated, from the performance charts for the aircraft, that adequate takeoff distance was available. After lift off the aircraft did not climb as the pilot expected. He then turned the aircraft left to avoid trees at the end of the strip. In doing so the left wing struck a dead tree that the pilot had not seen. A 10cm wide hole was torn in the undersurface of the wing from the leading edge to the aileron. As the aircraft continued to perform adequately the pilot decided to divert to the more suitable airfield at Coolangatta, 30 km to the north. The subsequent landing was without incident. The wind velocity was south-south-easterly at 10-15 knots and the takeoff was conducted into wind. There is high ground to the north-west of the strip and under certain conditions downdraughts are present in the area of the strip. However, on this occasion the prevailing wind was not conducive to their formation. The reason for the lack of climb performance was not determined.