Philbky
You are right AKL is not slot constrained, but it just so happens that BNE MEL and SYD are.
There's about a 9hr window for arrivals and departures in and out of DXB that connect Europe with OZ.
So rather than have the aircraft sitting around on contested ramps attracting absorbatant parking fees they fly them 3hrs to AKL clean them and fly them back to OZ dumping seats on the market and code shares with QF.
This way the passengers leave Europe arrive in OZ at the right time and vs versa.
Cheers Enos