I found this question difficult too, i did my degree in English so was able to comply with A2. My 16 yr old son though doesn't fit any of the categories as he hasn't yet finished grade 11. So I had to fill in the box with my justification that his mother tongue is english, he has done all his schooling in english up until now.
I've copied the requirements below:
English language
Principal applicants
The principal applicant is the person making the application. If you are the principal applicant, you must meet our standards of English. You can show us your ability with one the following:
A recent International English Language Testing Systems (IELTS) certificate, with a band score of 6.5 or better in the General or Academic modules. This certificate must be less than two years old, and is the best evidence of your English language ability.
A recognised qualification from a course taught entirely in English.
You are in ongoing skilled employment in New Zealand, and have been for at least the last 12 months.
Other proof of competency in English. We will consider a number of factors.
Non-principal applicants
This is anyone else on the application aged 16 and over. We accept the following proof of ability:
You have a recognised qualification from a course taught entirely in English.
You are in ongoing skilled employment in New Zealand, and have been for at least 12 months.
You have sufficient schooling in English:
primary and at least three years' secondary schooling,
five years' secondary schooling, or
tertiary course of at least three years' length.
You have certain secondary certificates with passes in English classes:
General Certificate of Education (GCE) 'A' levels from Britain or Singapore,
Cambridge Certificate of Proficiency in English,
Hong Kong Advanced Level Examinations (HKALE) including Use of English,
University of Cambridge in collaboration with University of Malaysia, General Certificate of English (GCE) 'A' levels,
International Baccalaureate, full diploma in English Medium,
STPM 920 (Malaysia), with A or B pass in English Literature,
South African Matriculation Certificate, with minimum D pass in English (Higher Grade),
South African Senior Certificate, with minimum D pass in English (Higher Grade), endorsed as 'matriculation exempt', or
New Zealand tertiary entrance qualification gained on completing year 13/seventh form.
You have a recent IELTS certificate with a band score of 5 or better in the General or Academic modules.
In all cases, we may still require an IELTS certificate.
If, as a non-principal applicant, you do not meet these standards, you can still apply if you pre-purchase ESOL training, which will let you improve your English after you arrive.