"Under the Care of Children Act 2004 and the Code of Health and Disability Services Consumers’ Rights 1996, the Health and Disability Commissioner1states “a child may consent themselves [to health treatment] if and when the child achieves sufficient understanding and maturity to understand fully what is proposed”.
van Rooyen, A., Water, T., Rasmussen, S., & Diesfeld, K. (2015). What makes a child a'competent'child. The New Zealand medical journal, 128(1426), 88-95.