The Piper Arrow was on a flight from Bankstown to Albury under the visual flight rules. At Goulburn, the pilot elected to return to Bankstown due to low cloud. During the return flight, the pilot noticed the engine oil temperature and fuel contents indications drop to zero. He transmitted a PAN call but received no response. Assuming he had suffered a radio failure, the pilot continued to Bankstown and joined the circuit on downwind for runway 29R. Bankstown tower controllers, observing the aircraft approaching with the landing gear retracted, unsuccessfully attempted to contact the pilot by radio. They then flashed a red light signal to indicate to the pilot he should continue circling. The common crash call and crash alarm were activated, and a full emergency declared. The aircraft subsequently touched down with the landing gear still retracted, skidding along the runway for about 70 metres before coming to a halt. The pilot vacated the aircraft without injury. The pilot subsequently reported that when he lost radio contact he assumed he had suffered a radio failure, although the radio continued to generate static. He continued his return and made a non-radio approach to Bankstown. After joining the circuit on the downwind leg, he selected the landing gear down but did not monitor the landing gear warning lights as he was concentrating on a slower aircraft ahead, and looking for light signals from the control tower. He did not see the red signal light and failed to realise the landing gear was not extended until he landed. A maintenance check of the aircraft did not find any defects associated with the electrical, radio or landing gear systems which may have contributed to the accident.