If you're going to be studying in New Zealand for longer than 3 months you are required to have a student visa. In order to get one you need:
- To be enrolled in an approved course at an accredited New Zealand university
- To pay a non-refundable application fee
- Evidence of payment of tuition fees or exemption from that fee
- A recent passport sized photo
- A written guarantee from an institution/person that suitable accommodation is available for you in New Zealand
- Evidence of sufficient funds to live on throughout the year
- A return air ticket to your country or evidence of sufficient funds to buy one
In order to study there you'll also need a passport which is valid for at least 3 months after your planned date of departure from New Zealand.
You'll also need to provide a medical certificate to show you meet an acceptable standard of health. If your course is longer than 24 months, you'll have to provide medical and x-ray certificates which are less than 3 months old. Also, if you've spent a combined total of 3 months or more in a country or countries which are not identified as being low risk of tuberculosis, in the last 5 years, then you'll need to have a TB screening.
If you're over 17 and planning to stay in New Zealand for longer then 24 months, you have to provides a police check to show you are off good character.