I did this myself - flew G reg in the UK on NZ PPL for a while, but then converted to UK PPL - this was about 9 years ago but all it took was a General Flight Test - more or less like a BFR - a little longer perhaps.
I am not 100% sure, but I don;t think that you can fly a G reg aircraft on NZ PPL outside the UK, so if you wanted to head over to France, that might be outside limits. No doubt BEagle can correct me if I am wrong on this point.
Cheers