Here it is
VH-EAS Avro 691 Lancastrian
My hunch would be that it was bought principally to maintain pilot currency and training, as well as making pioneering route trials. The BOAC Lancastrian fast service in 1947 to Australia required four aircraft (minimum) to hold down the thrice-weekly service, and the BOAC timetable states that it is "operated by Qantas" east of Karachi. This was just a continuation of the pre-war practice on the Empire Flying Boat service on the same route, which had also used Qantas crews on the British aircraft beyond Karachi. Qantas had its own fleet of Empire Boats, but to keep the crews up to currency on Lancasters must have been difficult without one. Notably the attached link states the aircraft was written off in 1949 while on a training detail.