Whatever applies to your long haul segemnt should apply to the short domestic connection if on the same ticket, in my experience. So if you dont have to pay extra baggage fees at united check in, then that should carry over to the QF recheck in at SYD. Also if you do have to pay united extra fees you should keep th receipt as that fee applies to the whole trip, if on one ticket. Hope you have a fun trip BNE!