Presuming you are buying the cheapest fare SYD-MEL, you are also vulnerable to missing the flight and the ticket becoming worthless. QF often takes a hard line on this (not quite as bad as Jetstar, but close). Also you are not eligibile to use the QF transfer bus from international to domestic, so that adds 10 minutes to the transit time (and AUD 4.50 to the cost). If all goes well, this transit can be done in under an hour, but lots of factors could extend it to over 2 hours (mostly late arrival of the NZ flight). At keast the bags can be interlined as long as you do fly QF (and not Virgin or Jetstar).