I am surprised by the ruling, in Joyce’s case. It was a decision by the NZ govt to award citizenship to descendants of citizens - how could he have known? And his father lived during the time when there was an entirely different citizenship structure in the Commonwealth. This NZ thing looks like a residual right, rather than a proper citizenship.
A big part of the problem seems to be that New Zealand has started interpreting its own laws differently, so anybody with a right to be a New Zealand citizen is a New Zealand citizen by default.