QF25/26 seems to be the route they change a/c on first if there is a need. I'm guessing it is partly due to load and yield, partly due to there being other alternatives for pax if the a/c change results in lower capacity (eg move some pax to NZ or put them on flights via SYD or NAN or the MEL-LAX nonstop instead).
It really is a bit of a lottery as to what you get in terms of a/c.