You still haven't provided any details.
As I've mentioned Air NZ's downgrade policy is very clear and I don't think I've ever seen another FT thread ever (nor seen mention anywhere else) of Air NZ not abiding by that policy.
There has to be more to this story, yet you're not providing any more information. Should be pretty easy to tell us why Air NZ downgraded you because it was obviously a Trans Tasman flight - was it a change of aircraft?