During the approach to land at Ord River Station the Pilot in Command (PIC) discovered that throttle movement was restricted within the range 18 to 23 inches of manifold air pressure. After considering the various alternatives the PIC elected to return to Kununurra and attempt a landing. The PIC sought advice from the company Chief Pilot before attempting a landing. Following a practice circuit, to determine the minimum approach speed obtainable, the Chief Pilot instructed the PIC to repeat the circuit and to close the mixture at 300 to 400 feet on final approach. The PIC flew a slightly modified circuit, which involved a longer, flatter final approach and closed the mixture, as instructed, at 300 feet on final approach. Shortly after closing the mixture the PIC realised that the aircraft would not reach the runway and she attempted a landing in a grass area short of and to the left of the threshold. During the landing the aircraft collided with a ditch which had been hidden from the PIC's view by the grass. Movement of the throttle was restricted because a throttle linkage pin, which had been fitted incorrectly, was binding against a loose induction air box. The bolts holding the induction air box in place had not been secured correctly at the last servicing. Three of the four retaining bolts were found lying in the engine bay. The PIC was inexperienced in commercial operations and the Chief Pilot was known to exercise "positive" supervision over the company's operations.