Earlier in the afternoon the pilot had flown from Noosa to Toowoomba. After a short stay the pilot and his passengers boarded the aircraft a took off for the return journey. When the pilot selected the landing gear up it did not retract. He completed emergency checks and found that the hydraulic fluid level was low. The flight was continued to Maroochydore where engineering advice would be available. Enroute the pilot contacted his maintenance engineer and following the discussion attempted to replenish the hydraulic fluid tank with engine oil. This was not successful. The aircraft was landed with the gear retracted and sustained damage to the propeller and fuselage undersurface. All the occupants were uninjured. Subsequent inspection revealed that the left maingear door actuator had separtaed allowing the hydraulic fluid to escape.