I thought if all the flights are on the same ticket then OP's bags will be tagged to Perth when he checks in at Wellington airport, and transferred automatically in AKL? From the Air NZ website:
Domestic to international transfers
If you are connecting from an Air New Zealand Domestic Jet flight (and selected Link flights) to an international flight, you can check your luggage through to your final destination at your departing airport. After arrival in Auckland, your luggage will be automatically transferred.
The same does NOT apply on his return journey as he will need to clear the bags through customs in AKL.