Longest routes within OZ/NZ are;

ADL-AKL
SYD-PER
PER-CBR
BME-PER
SYD-DRW

SYD-PER on a 743 will probably give you the most comfortable ride if your doing it for the sake of miles and not to visit anywhere. SYD-PER-CBR makes for a nice run, or SYD-PER//BME-SYD is a nice open jaw leg.

Edited to note; If you're looking to maximise first class usage then MEL-AKL with QF and AKL-SYD on LAN would be good, perhaps combine the SYD-PER-CBR run for max miles, get a cheap Virgin Blue/Jetstar flight from CBR-MEL and then MEL-AKL-SYD for the first class credits...