The pilot was conducting a bank-run flight. It was raining heavily when the aircraft arrived at Canberra, and the pilot took shelter while freight agents attended to the load. When this operation was completed, the pilot ran back to the aircraft, quickly boarded and prepared for DEPARTURE. Although Runway 35 was the runway in use, he was given a taxy clearance for DEPARTURE from Runway 12, as the cloud base and visibility were better in that direction, and the wind was light and variable. It was still raining as the takeoff roll was commenced, with water accumulation on the runway retarding the acceleration of the aircraft. At the start of the roll the pilot heard a tapping noise coming from the left hand side of the aircraft, which he thought may have been a strap from his seat belt hanging out of the door, but a quick check confirmed that his seat belt was correctly fastened. As speed increased the noise subsided, and as both engines appeared to be operating normally, the takeoff was continued. However, at the point of rotation the noise returned, louder than previously, and the pilot suspected that there was a fault in the left engine. As it was apparent that the aircraft would enter cloud shortly after becoming airborne, the pilot made a decision to abandon the takeoff. The aircraft touched down approximately 550 metres from the end of the runway, and during the application of heavy braking the aircraft aquaplaned along the wet runway. After crossing the threshold it continued through the boundary fence, across a road, then came to rest in an adjoining paddock. A length of the cargo restraint webbing was found hanging out of the cabin door, allowing it to flap against the side of the fuselage. Canberra Runway 12/30 has a known drainage problem during rain periods, and other aircraft have also experienced aquaplaning following abandoned takeoffs.