Is it possible to null an adoption in NZ?

If there is a procedural mistake, an illegal act or other flaw in the process a court may decide to nullify an adoption. The court would probably consider the best interests of the child and may condone a flawed process if the interests of the child demanded it.

Other than that, no. The adopted child, the biological parents and the adoptive parents cannot nullify an adoption. Adoption is for life.