I think I've worked this out - relevant FIFA requirements:
Article 17 Acquisition of a new nationality Any player who refers to art . 15 par. 1 to assume a new nationality and who has not played international football in accordance with art . 15 par. 2 shall be eligible to play for the new representative team only if he fulfils one of the following conditions : (a) He was born an the territory of the relevant Association ; (b) His biological mother or biological father was born an the territory of the relevant Association ; (c) His grandmother or grandfather was born an the territory of the relevant Association ; (d) He has lived continuously for at least five years after reaching the age of 18 an the territory of the relevant Association.
Wynne can't meet that requirement because he's not 23 yet. He hasn't lived in NZ for at least 5 years since reaching the age of 18. Therefore he isn't eligible for NZ?