Mogga, you maybe entitled to other citizenships but as long as you don't apply for them you are fine.
The situation with Joyce is silly, unless you knew that by NZ law you got NZ citizenship by decent automatically even though you where not born there or visited there you could go through life not knowing you are a citizen and NZ would not know you exist.
I imagine there are a lot of people around the world born to NZ citizen's overseas that have no idea they are suppose to be NZ citizens and of course NZ don't know who they are.
So I ask the question, how can NZ claim you as a citizen if they don't know you exist?