While enroute from Batchelor to Tindal, the pilot encountered deteriorating weather conditions and landed the aircraft in a suitable paddock, on Bonrook Station. The following day he inspected the landing area and judged the length to be 500 metres and that the area was suitable for takeoff. However, the weather conditions were still unsuitable for departure. The next day, after obtaining a weather forecast and being satisfied that the weather was suitable, the pilot decided to continue the flight. He and his passenger boarded the aircraft and the pilot configured the aircraft for a short-field takeoff. During the subsequent takeoff the pilot rotated the aircraft at 55 kts. It became airborne and commenced to climb clear of the 10 metre high trees at the end of the strip. However, the pilot became concerned about a tall tree directly ahead and commenced a left turn to avoid the tree. The stall warning sounded, one wing dropped and the aircraft impacted the trees. The pilot suffered minor lacerations to his head, probably as a result of being struck by sections of the broken windscreen, and the passenger received minor bruising. Checks of the fuel did not reveal any contamination and the pilot believes that the engine was operating normally during the takeoff. An inspection of the takeoff area revealed that the surface was firm and that the grass had recently been slashed. The performance charts contained in the flight manual show that the strip distance available was adequate for a takeoff from a dry grass runway. However, on this occasion, it is likely that the surface was wet and covered in recently slashed grass and that takeoff performance was degraded. The pilot was not familiar with operation from such strips.