I thought the problem with this JQ setup was that SIN - DRW was classified and arrived at the International part of the terminal in DRW, then had a different flight number as a purely domestic flight on DRW - MEL - DRW. It may have been the same aircraft, but used two flight numbers.
Where as QF or Australian Airlines when they operated SIN - DRW - CNS was classed as an international flight and used the same flight number the whole way through.
I'm happy to be corrected if this is not how it is or was.