If you are flying to North America anytime in the future, try and purchase a ticket EDI-AKL-EDI-USA-EDI and then use the USA trip whenever you need to. I have done this before when I lived in the UK and made trips to both Dubai and the US in the same year so that I could get the piece allowance for my Dubai trips. If your J class ticket is restricted, things could get messy, but if its full fare this should work well.
Cheers,