It's very simple. New Zealand has signed up to a whole lot of United Nations treaties, and in one of them, the Convention on the Rights of the Child which we signed in 1990 and ratified in 1993, we guarantee the right of every child to have access to a free education.
Article 28a of the Convention of the Rights of the Child states that we will:
Make primary education compulsory and available free to all;
Our reservations when signing the treaty were:
Nothing in this Convention shall affect the right of the Government of New Zealand to continue to distinguish as it considers appropriate in its law and practice between persons according to the nature of their authority to be in New Zealand including but not limited to their entitlement to benefits and other protections described in the Convention, and the Government of New Zealand reserves the right to interpret and apply the Convention accordingly.
The Government of New Zealand considers that the rights of the child provided for in article 32 (1) are adequately protected by its existing law. It therefore reserves the right not to legislate further or to take additional measures as may be envisaged in article 32 (2).
The Government of New Zealand reserves the right not to apply article 37 (c) in circumstances where the shortage of suitable facilities makes the mixing of juveniles and adults unavoidable; and further reserves the right not to apply article 37 (c) where the interests of other juveniles in an establishment require the removal of a particular juvenile offender or where mixing is considered to be of benefit to the persons concerned.
Note the bolded part above, which means that the way the NZ Government has interpreted free education as allowing schools to charge donations in order to make it appear that schooling is totally free in New Zealand, even though those donations aren't eligible for a tax deduction and the schools expect the donations to be paid.
It's all a fiction, but one that must be maintained because of the UN Declaration that we signed. So, there is no way, as people are asking for on the radio, for these donations to be called what they really are, compulsory school fees for public education, because our education here is free! Doncha just love it!