I do not know if the “North island” would have a big advantage or not. There is same species more common down here, while other species (mainly Snapper, Blue Maomao, Kingfish and John Dory) are more common in the north. But for most species there is a degree of overlap especially during the Summer.
Wellington and Marlborough in same ways get a selection of both. While further south the fishing gets harder which is way I gave BP to Kingies and Snapper if they are caught further south.