The aircraft was on a test flight following maintenance. The pilot called Cairns Approach for clearance and a clearance instruction was issued. However, no acknowledgement of the clearance was received and the aircraft did not appear on radar when expected. When further attempts to contact the aircraft failed, search and rescue action was initiated. Approximately 40 minutes after radio contact with the aircraft was lost, the pilot telephoned Cairns Tower advising that the aircraft had suffered a communications system failure (including the transponder) and that he had landed at Cairns after following the loss of radio procedures. Investigation revealed that there had been a failure of the Cairns Approach primary radar around the time communications with the aircraft were lost. Cairns Approach was not aware of the failure and saw neither a primary, nor a secondary return from the aircraft on the radar displays. The Cairns Tower controllers were conducting a visual search in the expected approach sector of the aircraft and did not see it land. Primary returns are represented by a small cross which appears in the centre of the circular symbol representing the aircraft. The disappearance of this cross was the only indication to the controllers that the primary radar had failed. The size of the cross is such that its disappearance might not be noticed unless close attention was being given to that particular part of the radar display. Then factors considered relevant to the development of this occurrence were: 1. The aircraft suffered a communications system failure which included the transponder. 2. There was a failure of the Cairns Approach primary radar of which the controllers were not immediately aware. Safety Action. A SADN was submitted shortly after the incident concerning the primary radar failure and recommending the fitting of an alarm system to alert controllers of a primary radar failure.