There are ways to handle WI scenarios which involve the disappearance and non-existence of different ethnic groups and other populations. This include ethnic groups and other populations which have been subjugated or minoritized. I can imagine scenarios where the Welsh are as thoroughly assimilated as the Cornish, say, or ones where French is a dead language. I can also imagine scenarios where the proto-Maori never made it to New Zealand; navigation on the high seas is risky.
Key, I think, is making it clear that this scenario where one group or another does not exist is not a utopia. "If only these people never existed, things would be so much better!"
Turning to this scenario, I suspect that the New Zealand ecology will be thoroughly ravaged. Leaving aside salutary examples like terraformed Ascension, 19th century globalization tended to wreak havoc on isolated ecosystems. Absent someone remarkably conservation minded, New Zealand would differ only in scale.