SWH,
I think you'll find that Thai doing BNE_SYD_BKK means they can through carry the BNE pax to BKK, just as CX does/did ADL/MEL/HKG as well as BNE/CNS/HKG. It's not approval carry pax just between the domestic ports. I doubt QF is carrying domestic pax LAX/JFK.
Many airlines have had approval to do similar over the yrs.