On final approach, the pilot observed that the single gear down light (green) was illuminated. Following a smooth touchdown near the threshold, the aircraft rolled along for about 150 metres before the pilot noticed the left side of the aircraft had settled lower than normal. Suddenly the aircraft veered uncontrollably to the left and struck a T-vasis light box before coming to rest with the main gear partially collapsed. Post flight inspection revealed that the main landing gear had not been fully extended. Examination of the landing gear system revealed no evidence of pre-existing failure or damage. The gear collapse was evidently the result of inadequate maintenance and inspection of the system. This had allowed the gear down light to be illuminated when the maingear had not been locked in the overcentre position.