The pilot had intended to carry out a scenic flight which would include aerial photography of a property at Merrijig. An enroute landing was not planned, but the pilot subsequently advised that one of his passengers became unwell and a decision was made to land at a grass strip near the property. Take-off from this strip was commenced about 90 minutes later, and the acceleration was reported to be slower than expected. The aircraft was pulled into the air near the end of the strip, but it then descended, ran through two fences and collided with some disused farm machinery. It came to rest in a nearby river bed. Prior to the attempted take-off, the pilot had made calculations from the performance charts in the aircraft. However, the strip was 100 metres shorter than estimated, and the pilot also incorrectly assessed the take-off weight of the aircraft. Based on the corrected figures, the charts indicated that about 1030 metres would have been required for take-off. The strip being used was only 550 metres in length and, although it had a downslope, it was covered in grass some 20 centimetres high. The long grass had slowed the rate of acceleration of the aircraft and flying speed could not be obtained under the conditions.