Two different issues though.
No/few born & bred mainlanders in national age group teams, could potentially yes be due to some NZF bias/focus towards Auckland esp, or neglect of talent ID in the South Island.
However it's not job of Nix or Ole to look after South Island or even NZ football. With Ole if you are of a standard, and your parent's have the cash then I presume they are happy to take you on board even if you come from Bluff.
Is the new Christchurch Football Academy (that Russian businessmen Slava Meyn) at Yaldhurst aiming to provide high quality coaching etc like Ole?
If so you would expect kids from there will start to get noticed, at a national level.