You get an orange 'D' sticker on your boarding pass that indicates you're a domestic passenger. Walk straight through customs and immigration at both ends. You would have to recheck in SYD for your international sector.
If you want to transit internationally then you'll be treated as an international pax ex BNE and use the transit facilities in Sydney meaning no customs formalities, just a security a re-screening (can't believe I said just, it's a pain in the arse).
Or just do like every other QF pax does and buy a ticket with EK or SQ ex BNE - far better option these days.
