On entering downwind for runway 06 the crew of VH-FKA saw an aircraft on the taxiway. The aircraft was then observed to enter the runway, backtrack and take off on runway 06. No communications could be established with the departing aircraft on either the area frequency or the Mandatory Traffic Advisory Frequency (MTAF). There was no loss of separation. With the assistance of company personnel on the ground the crew of VH-FKA were able to establish the registration of the departing aircraft was VH-PMT. Significant factors: The following factors were considered relevant to the development of the incident: 1 The pilot of VH-PMT did not follow the radio communication procedures for operation in an MTAF. The reason for this was not determined.