You are totally missing the point. Timana Tahu is a perfect example. His ancestory is Maori. He is not a New Zealander. Again, I know a lot of Australian born people who are from Maori heritage. They associate with their heritage (their race), but acknowledge themselves as Australians. It's their right, their prerogative, and there is absolutely nothing wrong with it. That's just how it is. Maori is not a nationality. It is a heritage, a race. Again, Tahu is an Australian with Maori race ancestory.
No, you don't have to be Australian to represent U19s. Infact, that's quite naive to suggest you do. It is based on residency. It is not a citizenship criteria. It is more a criteria of you are at an actual age and you reside in a particular area. Playing for a national team means you are that nationality, you can live elsewhere, and you can be any age, but you are that nationality. Much like schoolboys, Benji and the like played Australian schoolboys, because, in actual fact they WERE Australian schoolboys. As in they were going to school here. But that does not necessarily make them Australian. Again, Benji was an Australian schoolboy, but he is not Australian. Timana Tahu is of Maori heritage, but he is not a New Zealander. Both are right to have represented NSW U19s, Australian Schoolboys, and now for Tahu New Zealand Maori.