NZ has obligations to the UN
As a signatory to the United Nations Convention on the Rights of the Child, New Zealand is obliged to ban smacking.
Background on the Convention on the Rights of the Child from
UNICEF:
"Built on varied legal systems and cultural traditions, the Convention on is a universally agreed set of non-negotiable standards and obligations. These basic standards—also called human rights—set minimum entitlements and freedoms that should be respected by governments. They are founded on respect for the dignity and worth of each individual, regardless of race, colour, gender, language, religion, opinions, origins, wealth, birth status or ability and therefore apply to every human being everywhere. With these rights comes the obligation on both governments and individuals not to infringe on the parallel rights of others. These standards are both interdependent and indivisible; we cannot ensure some rights without—or at the expense of—other rights.
A legally binding instrument
The Convention on the Rights of the Child is the first legally binding international instrument to incorporate the full range of human rights—civil, cultural, economic, political and social rights. In 1989, world leaders decided that children needed a special convention just for them because people under 18 years old often need special care and protection that adults do not. The leaders also wanted to make sure that the world recognized that children have human rights too.
The Convention sets out these rights in 54 articles and two Optional Protocols. It spells out the basic human rights that children everywhere have: the right to survival; to develop to the fullest; to protection from harmful influences, abuse and exploitation; and to participate fully in family, cultural and social life. The four core principles of the Convention are non-discrimination; devotion to the best interests of the child; the right to life, survival and development; and respect for the views of the child. Every right spelled out in the Convention is inherent to the human dignity and harmonious development of every child. The Convention protects children's rights by setting standards in health care; education; and legal, civil and social services.
By agreeing to undertake the obligations of the Convention (by ratifying or acceding to it), national governments have committed themselves to protecting and ensuring children's rights and they have agreed to hold themselves accountable for this commitment before the international community. States parties to the Convention are obliged to develop and undertake all actions and policies in the light of the best interests of the child."