When the aircraft reported at the position reporting point after Mt Isa, it was found that it was not tracking on the route expected by air traffic controllers. There was no confliction with other aircraft and the aircraft was recleared along the route it was tracking. Earlier in the day the original flight plan was submitted to track via air route A575 after passing Mt Isa. This plan was later cancelled and another submitted to track via air route A464 after Mt Isa. The first route is the preferred tracking and the amendment resulted from the possibility of adverse weather on air route R575. Prior to departure from Brisbane information indicated that weather was now suitable on R575. The airline company operations personnel checked if the Brisbane Briefing Office held a flight plan on the aircraft, and when they were advised that the office did not hold a plan another plan was submitted for the aircraft to track via air route R575. However, as the first two plans had been submitted by the airlines base in Bangkok through Melbourne (normal practice), Brisbane was unaware of the existence of a plan for the aircraft. As a result, a cancellation message was not sent prior to the third plan being submitted. Flight Progress Strips are prepared for each plan by two sections in the Brisbane control centre and distributed to controllers who will become responsible for the control of each aircraft. On this occasion, when the third plan was submitted it was assumed that it was a duplication of one of the previous plans, probably because no cancellation message was received, and no strips were prepared for the controllers. As a result, the controllers assumed that the aircraft would track via air route A464 after Mt Isa. The controller who received the Mt Isa position report marked the information on the strip for the next position reporting point along air route A464 and not along R575, the point that the aircraft advised it would report at next. He then co-ordinated the Mt Isa position with the next controller responsible for the aircraft. This controller also had strips for air route A464 and was not aware the aircraft was tracking via air route R575 until the next position report from the aircraft.