A Cessna 208B Caravan, registered VH-NTQ, was en-route from Broome to Koolan Island, Western Australia (WA) at an altitude of about 9,500 ft, when the pilot noticed a drop in the engine torque indication, with a corresponding drop in the engine oil pressure indication. The pilot diverted to the nearest airstrip, which was Beagle Bay, WA. The pilot shut the engine down when the low oil pressure warning light illuminated and conducted a landing at Beagle Bay airstrip. The aircraft overran the airstrip, coming to rest upside down after impacting a mound of dirt. The aircraft was seriously damaged. The pilot, who was the only occupant, sustained minor injuries. From the evidence available it was evident that the engine had a substantial in-flight oil leak, which necessitated the in-flight shut down of the engine and a diversion to the nearest available airstrip. The accident damage to the engine in the area of the apparent oil leak precluded a conclusive finding as to the source of the leak. Although the detailed examination of the oil tube attachment lug fracture surfaces was inconclusive, the oil tube remained the most likely source of the oil leak. Evidence from other oil tube failures indicated that significant vibratory loading can cause the oil tube attachment lugs to fracture in the manner observed in the oil tube fitted to VH-NTQ. There was no evidence that the transfer tube was subjected to vibration from a compressor turbine or power turbine blade failure or of an incorrectly fitted engine mount. There was also no evidence of a pre-accident defect that would have caused a reduction in actual engine torque.