Since we started this discussion I have found a wide variety of references to how many Lancastrians Qantas had, from one (my original opinion from two separate sources, both hitherto reliable) to nine, with various numbers in between as well. I presume they were not all in the fleet at the same time.
Here's the BOAC timetable from September 1947 which shows the thrice-weekly Lancastrian service to Australia, with just a note that the sectors beyond Karachi are operated by Qantas, which takes a minimum of four aircraft, I presume all BOAC, to hold down the schedule, and doubtless several more with the extended maintenance and unserviceability that was standard at the time. The implication is through aircraft. Presumably with all those crew slips that would be needed (unlike the flying boat services which stopped overnight and kept the same crew for several days) it was better to crew from both ends.
http://www.timetableimages.com/ttima...47/ba47-05.jpg
BOAC bought Lockheed Constellations with scarce foreign exchange at the time on the justification that they needed them to compete with US carriers on the Transatlantic routes, but the Connies were almost immediately extended to the Australian route as well, as there was a significant passenger (and foreign exchange) loss occurring, especially from the Australian end, to Pan Am on better aircraft to go from Sydney to London via the USA.