On return from the training area the pilot advised that he had an unsafe indication from the nose landing gear. The tower confirmed that the nose gear was in the retracted position so the pilot elected to return to the training area to attempt to rectify the problem. All attempts to lower the nose gear using normal and emergency systems were unsuccessful. Emergency services were placed on standby and the aircraft subsequently landed with the nose gear retracted. Investigation revealed that the nose gear door actuating system was worn excessively and out of rigging tolerance. This allowed the system to go to an overcentre position and hold the doors in the closed position, thus preventing the nose gear from extending. The door actuating rods were incorrectly attached to the rear side of the door brackets, and the door hinges were stiff from lack of lubrication.