The pilot reported that he had accumulated about 6.5 hours flying for the day as well as having spent time on the ground on business at a number of locations. It had been very hot and turbulent throughout the day. He was relieved when he reached the Newman circuit and relaxed as the flight was nearly over. He joined the circuit on downwind and made a general downwind broadcast on MBZ frequency. The pilot thought he had completed the downwind checks correctly but subsequent events indicated he had forgotten to lower the landing gear. During his finals checks he noticed that the landing gear lights were not illuminated but he assumed he had forgotten to turn the brightness back to the daytime setting. He realised the gear was still up when the propeller started to hit the ground. The landing gear warning horn, which had not sounded during the circuit, came on as the aircraft settled onto its lower fuselage. The pilot reported that the landing gear waring horn may have been intermittent prior to the accident. He had made a note on the maintenance release to have it checked at the next periodic service which was due when the aircraft arrived back in Perth. The pilot indicated that he felt that the factors leading to the accident were very straight forward. He was fatigued after a long day and had relaxed too much when he reached Newman. He had not paid sufficient attention to the operation of the aircraft and his fatigue had led to him rationalise the lack of landing gear lights as a rheostat setting rather than an indication the gear was still up. The failure of the landing gear warning horn to sound aggravated the situation.