If they want to be anno - friend of mine has a Maori dad, she does not herself see herself as Maori. Little welsh/kiwi baby girls. 
Another friend has Maori mum and dad was from England. She does see herself as Maori and told me feels her ancestors are always with her. Her husband is white kiwi and her brother is half Maori half Japanese. It is hard to keep up sometimes. And impossible to tell looking at someone whether they consider themselves Maori or not.
In some areas there are financial advantages to being Maori - it all goes back to the treaty of Waitangi , land rights, fishing rights etc.
I guess Kiwis can also tell the difference between Maoris, Samoans, Fijians etc - but this is tricky. If they have a great big grass mat tied like a wide round their mid sections that is a clue (Samoan I think, not Maori). Tattooed chin in middle aged women - yes! Maori elder. Easy.
But I look the All Blacks and can't guess. (gazing on the All Blacks is never arduous though is it)
I guess Uk people can often tell the difference between West Indians and Africans and kiwis would struggle.
Oh if you've got a name like All Black Ma'a Nonu - apostrophe in the middle - that's Samoan.