The biggest problem with the TAA A300s was their short range (due to being early model B2's - later A300s had significantly better range). Winter SYD-PER services regularly diverted to ADL due to their inability to carry sufficient fuel reserves to reach PER direct. On the east coast, they were very successful aircraft with the capability to carry significant amounts of cargo in addition to a full load of pax & baggage.
When QF bought TN, they were able to compare the operating costs of the A300 against the B767-200 & -300 which didn't come out favourably for the A300. Apart from the huge disparity in range, the A300 burnt approx 1 tonne per hour more than the B763 and the cost of spares and engineering support was horrendous because of the small fleet size. The figures that James Strong gave in 1995 was that the four A300s had $10m per aircraft in spare parts whereas the eighteen (at that time) B747-400s had only $3m per aircraft in spare parts. He went on to say that the minimum number of aircraft for a fleet to be viable was 12-15. (this was confirmed later when QF bought Impulse which had a fleet of 8 B717s & B1900s - QF was able to lease 6 more B717s without any increase in the spare parts inventory).
In 1995, Strong wanted to get rid of the 4 A300s ASAP but because the TN Board had overvalued all of its assets in preparation for the Federal Governments sale of Australian Airlines, the book value of those aircraft was significant and Strong said that shareholders would not accept writing off that amount in one financial year (Continental Airlines in the US had just parked 30 A300s in the desert so there was no market to sell them to another carrier). The aircraft were placed on an accelerated write-down so that they would be worth nothing by mid 1998 - the aircraft left QF service circa August/September 1998. During the last few years of A300 operations, the serviceability rate decreased (mainly Landing Gear related) which may have been due to QF reducing the spares inventory.