Qantas, like most if not all operators only use as much thrust as required for that specific takeoff without creating a limiting situation at any time. They use what is called Flex Thrust [airbus] or Assumed Temp [boeing] to calculate what would be the minimum thrust required to achieve a safe takeoff. Obviously, the less weight you have at takeoff on a specific runway, the less thrust required. If this flight was departing for LHR instead of SYD the BRW would have been considerably higher requiring a higher thrust setting & much closer to the max certified thrust of 72,000 LB. This higher weight could have brought about a totally different outcome to what eventuated, especially when you take into account that the previous two sectors were WSSS to EGLL & return.
To the best of my knowledge the TRENT 900 is capable of Thrust limits of 80,000 LB which could be bought by any Airline as required, at a price from RR. Each Airline only buys the certification needed & this certification is limited by a simple selection by RR with the computers. As QF only operate the engine at 72,000 LB the engine is already operating 10% below it's maximum design limits. The reason for the possible higher certification limits is to cater for the freighter & future stretch should this ever happen.
Hope this helps.
P B