VH-JJT was on departure from Ayers Rock climbing through 4000 feet and intercepting the outbound track for Alice Springs when the crew spotted a Cessna in their 2 o'clock position about 800 metres distant. The Cessna was at the same altitude and apparently on descent into Ayers Rock. No avoiding action was necessary but prior to the sighting the crew of JJT were not aware of the presence of the Cessna. They also believe the Cessna was unaware of their presence. At the time of the occurrence there was a tour group of aircraft at Ayers Rock and the MTAF frequency was being used by tour organisers to arrange aircraft parking and for other non operational purposes. This meant that the frequency was extremely busy. There were a number of other aircraft in the area at the time and the crew of JJT reviewed the traffic situation before take-off but they did not have information on the Cessna that they conflicted with. Either the Cessna did not make an advisory call or the crew of JJT missed any call that may have been made. It is also possible that any call from the Cessna was over-transmitted by another source. Significant Factors The following factors were considered relevant to the development of the incident: 1. The crew of JJT were not aware of the presence of the Cessna. 2. The MTAF frequency was extremely busy. 3. Either the crew of JJT missed any advisory call made by the Cessna pilot or no call was made.