Unfortunately QF is requiring that both flights must be on QF to use the free bus; you must arrive in SYD on QF as well as depart on QF, and they do check. I guess the operators of the paid buses complained at losing their revenue if QF was taking other pax. Also to use the bag drop after customs requires having tagged the bags through in AKL, otherwise they won't take them there and require you to lug them to the domestic terminal and check-in landside. So there is interlining, even with the customs and agriculture check at SYD.