The pilot reported that after take-off the landing gear could not be fully retracted. On arrival at Dubbo efforts to lower the gear by normal and emergency means were unsuccessful. The pilot elected to divert to Bankstown and subsequently landed on the partially extended gear. The nosegear collapsed and the aircraft slid to a halt. The brush pigtail in the landing gear electric motor had failed through normal use. The pigtail and its soldered washer had allowed massive arcing to occur, causing the motor and the actuating shaft to seize. The landing gear had been in an intermediate position when this occurred, and because the emergency lowering system is connected to the same actuator shaft, it also became inoperative.