I checked an ABC of jan. 1992, when CX still had their Tristars, and there were no scheduled Australia Tristar flights then. Melbourne, Perth and Sydney were done with 747s, while Cairns, Darwin and Adelaide had/have no direct flights. Australia could not be reached by Tristar (they didn't have the long range 500s) without stops so I guess the 747 was the best choice. But probably the Tristar was OK for (military, government, leisure) charters to everywhere.
nobody has ever died from hard work, but why take the risk?