I may be many years out of the new zealand school system now, But i'm not sure this is too realistic. I'd wager that by this point, even if they are very fragmented, there are two many whites in the North Island to be so easily undone. Northland is under control as is the Waikato. New Zealand political figures were VERY adept at convincing London of the importance of supporting the cause of settlement. A british pull-out is simply not going to occur. With ongoing population collapse in Maori, ongoing british/irish immigration, the number of people already in Auckland, a decisive maori victory this late in the game is impossible. By the moment the british move into the waikato, maori independence is a dead cause.
By 1870, just after this period there are a quarter of a million whites in the country, most in the south island but I'd wager those in the north are already a majority there.
also, you're not supposed to say "Maoris" unless you're an old white person from Whangarei. It's just Maori.