Ethnically, they're all Polynesian. There's no debate in the matter, it's anthropologically proven and I don't know any Maori who'd debate it. And it's not thousands of years of isolation- it's around one thousand.
Now culturally it's a different matter. If what you are saying (and I think this is the point you're trying to make, you've just got your terminology mixed up a bit) is that Maori consider themselves somewhat separate to Pacific Islanders, then you're absolutely correct- they do, especially in NZ where they're the indigenous people and other islanders are mostly recent immigrants. But if asked they won't deny the fact that going back long enough, maori, cook Islanders, Samoans, Tongans are all descended from the same ancestors.
That said, I think it's a valid point that combining Maori with Pacific Islanders in a combined side doesn't make much sense- it's like combining all Europeans in one side.. A combined pacific Islands side (featuring the islands, both Melanesian and Polynesian but not NZ) makes more sense due to their closer cultural ties to each other over recent history.
