A fine may seem harsh, but he is required to obey the laws. Ignorance is no excuse. NZ may operate zero tolerance on this for all we know.
It's also the law in the UK - here the fine is usually £60, but can be up to £500. Drivers are responsible for ensuring children are wearing seatbelts and adults are responsible for their own. I usually remind visitors to my country this is the case. It's now a habit for me to wear a seatbelt, regardless of the law wherever I am or which seat I am in in a car. Good habit to get into. May be because I am so used to wearing them, I'm wondering why the OP didn't wear them, given the safety benefits.