CX can't sell domestic legs in isolation.
But the point here is, only for flight above tropic of Capricorn(toc), which probably limits all flights to ROC, (if north of TOC), MKY, TSV, CNS. DRW, BME.
Who would want to fly internationally into one of these ports via another of these ports ?
Maybe some Indo carrier from DPS to ROC, MKY or BME once a week in a B737 or A320 via DRW, where they could also sell the domestic leg in isolation.
Don't think there would be much demand except if they did FIFO flights DRW/some port in WA that could handle that size of aircraft.
Not talking anything south of TOC.
FYI
have flown on a NH 744 & a PX 763 on a BNE/SYD sector on a Ansett/Qantas ff ticket, but don't think this type of thing happens anymore.