Adoption is a privilege ; not being discriminated against by adoption groups is a right. So an adoption group has to be able to justify who gets what priority without resorting to discriminatory reasons (or thinly veiled equivalent to discriminatory reasons - "no maternal presence" is still discriminatory), but doesn't have to provide a child if there are no child to be provided.
For the Maori, I'm not familiar with specifics of New Zealand-Maori relations, but if they have faced the same kind of treatment other indigenous groups in the Commonwealth and the US have (eg, repeated forced removal of children, through residential schools then adoption, to raise them outside their culture, in many case still ongoing) then the adoption of Maori children should be a wholly separate question from other adoption, and Maori communities should be in charge of making any calls that need to be made for how these children are to be fostered or adopted, including having the right to prioritize keeping those children within their community and culture - this should not be considered discriminatory. There's similar points to be raised depending on how much of a degree of self-government the Maori have (if they're reasonably self-governing they should have their own child welfare system, that should likewise be able to prioritize keeping the child within their community).
