Shortly after take off, the pilot advised ATS that aircraft operation was abnormal and he was returning to Port Lincoln. The pilot requested emergency services be placed on standby as the aircraft had a hydraulic problem and he was unable to extend the landing gear normally. After successfully carrying out a manual extension of the landing gear the aircraft was landed safely. A maintenance investigation revealed a cracked flared fitting in the hydraulic system, which allowed the system fluid contents to be lost. The fitting was changed and the aircraft returned to service. The fitting is the subject of a Defect Report to CASA.