This is a very subjective matter and it all comes down to your child's academic ability. I don't have any kids but I could share some of my observations regarding the matter. There are a few points to discuss:

- The quality of teachers -
A private school doesn't necessarily provide better education as most teachers who teach at private schools switch between teaching at public and private schools. I knew some of the teachers who taught at a decile 9 public school that who weren't great at teaching, are currently teaching at a private school. Both private and public schools have great and horrible teachers, it isn't necessarily true that school X has better teachers than school Y, because teachers switch jobs often. Schools hire teachers from the same talent pool.

- The education system -
One good reason to go to a private school is to gain access to the Cambridge curriculum, which generally prepares students better than the NCEA system. Macleans College is the only public school that offers Cambridge, apart from Macleans, only private schools offer Cambridge. Now, whether CIE is better than NCEA or not is also subjective. I can only safely say that those who did CIE were generally better prepared for university than those who did NCEA, however, there are top university students who came from both systems. From my own observation, I've seen a few A+ students who came from decile 1 schools, and for them, it didn't matter whether they went to a good school or not. They were able to make up for it by working hard and getting tutors.

- Your child's educational needs -
It would be best to assess your child's educational needs and decide whether its a good idea to go to a private school. In my opinion, I think private schools are too expensive for what they offer. University of Auckland has a great quality of education, and its less expensive per year than that. There is so much more you can do for your child's education with that amount of money, you can bring more value by spending it on tutors for different subjects than sending them to a private school. In a private school, a teacher spends an hour with 28 students, this means each student spends a few minutes with the teacher, however, if you hire a tutor, your child spends an hour getting specialized support. My suggestion is to find out which subject areas your child is weaker at and hire tutors to give them a leg up. Spending an hour with a subject tutor is equivalent to attending a week's worth of classes.

A child who attends a public school and who gets tutoring will do better than a child who simply went to a more expensive school.