After the student pilot had completed 1.2 hours of dual instruction in the circuit, his instructor assessed the pilot was ready for his first solo flight. The aircraft was brought to a halt on the grass runway 09 where the instructor alighted. The subsequent takeoff roll appeared to be proceeding normally when the aircraft suddenly veered to the left, departed the runway and collided with a boundary fence. The student pilot advised that the right cabin door had opened and he lost directional control when he lent over and attempted to close the door. The instructor reported that the right hand door lock was operating normally at the time of the accident. The accident was not the subject of an on scene investigation. This report is based on information supplied by the pilot.