HA! I think we've all used rocks at some point!
I'm not familiar with New Zealand rigs (will have to do some googling) but I have had the opportunity to tinker with a few pack saddles (mostly sawbucks and deckers, just a couple formfitters).
The main reason I want a formfitter is that the formfitter panels have flex in them and seem to to be more forgiving of not having a perfect fit to the beast's back. Where the trees on a sawbuck stay rigid the formfitter panels flex to accommodate the changes in the animal over time as it becomes more conditioned, or for use on more than one animal.