Constitutional law is the law that governs New Zealand's public administration and the relationship between the individual and the state.
Like the United Kingdom, New Zealand has no written 'supreme' constitutional charter that establishes the fundamental laws, formally establishes and empowers the institutions of government, and guarantees the civil and political rights of citizens. New Zealand operates under an informally organised framework of rules that establish and empower the three branches of government and provide for the rights and duties of citizens. This framework consists of imperial and legislation, the common law, and constitutional conventions and practices that are binding in a political rather than legal sense. These rules describe the exercise of public power among the instruments of the state and embody all the elements of a legally established and regulated system of government.
Because New Zealand lacks an identifiable constitutional charter that delineates the scope of constitutional law, the boundaries of New Zealand constitutional law have never been precisely drawn. Constitutional rules may be changed by less formal means than under a written constitution, and the spheres of public and private law are merged rather than separated.
Source: The Laws of New Zealand - Constitutional Law
A Constitution for Aotearoa New Zealand by 
Constitutional and Administrative Law in New Zealand by