The pilot of a Cessna 182, VH-EWV, while taxiing for departure at Ayers Rock, turned into an unserviceable taxiway in order to conduct engine checks before takeoff. The certified air ground radio operator at the aerodrome asked the pilot if his operations were normal, as this was not a normal procedure. The pilot confirmed that his operations were normal. A Cessna 207, VH-UAA, passed the other aircraft on the taxiway, and continued onto the runway. The pilot of UAA broadcast his intentions and started the take-off run. The broadcast was heard by the radio operator, but not by the pilot of EWV, who also broadcast his intentions to enter the active runway. This broadcast was not heard by either the radio operator or the pilot of UAA. As EWV approached the runway, the radio operator instructed the pilot to hold position. The pilot did not hear the instruction, however, on seeing the other aircraft moving toward his position, stopped immediately near the runway strip. The pilot of UAA continued the takeoff, passing at high speed and in close proximity to EWV. The role of the certified air ground radio operator was to provide information, not to give instructions, however the radio operator considered his actions to have been appropriate. The investigation found that the pilot of EWV understood the procedures to be used at the aerodrome. However, it was not possible to determine why communications to and from the pilot were not heard.