The pilot advised that a normal touchdown was made on the mainwheels. However, shortly after the nosewheel was lowered to the runway the right gear leg collapsed. The aircraft subsequently swung off the side of the runway. There were no external witnesses and all four persons on board the aircraft reported that the landing was normal. Inspection of the aircraft showed that the right maingear leg had collapsed rearwards, consistent with the application of upward and rearward forces which exceeded the structural strength of the gear attachments. It could not be established whether the overload forces had occurred during this landing, or whether a heavy landing on a previous occasion had resulted in the weakening of the gear support structure.