The aircraft was engaged on a charter flight with the first three sectors having been flown without incident. On arrival in the circuit at Bamyili, the landing gear was selected down and although the pump motor was heard to operate, the maingear could not be seen and the green "gear down" light did not illuminate. The landing gear was subsequently cycled and the gear would neither lock up nor down. A emergency manual extension was attempted and at this time a total absence of pressure in the system became apparent and it was established that there was no hydraulic fluid remaining. Radio discussions were held with company engineers and an attempt was made to manually extend the gear with the passenger pouring engine oil into the system while the pilot operated the manual pump. This attempt was unsuccessful as the fluid was lost before pressure could be built up in the system. The pilot decided to return to Tindal for a visual inspection of the landing gear position, which revealed that the nosegear was "down" and appeared locked but the maingear legs were dangling just out of the wheel wells. Adelaide Flight Service was notified with a "Pan" call and preparations made for a gear-up landing on the grass strip to the left of the main runway. After a final attempt to retract the nosewheel, the aircraft was configured with reference to the operating handbook, the emergency services were put in place and the passenger briefed. After several practice approaches, the engine and electrical system were shut down and a landing was made, with damage being confined to the propeller and underside of the fuselage. Subsequent investigation revealed a failed seal had allowed all fluid to escape from the landing gear hydraulic system.