New Zealand customs are happy if you tell them you have 3 months of medication.
It's a good idea to have a letter from your doctor listing your medication, though in 40 years of travelling I've only needed it once. If you need to get extra medication in New Zealand it'll cost you somewhere between $50 and $100 (I think $50 but I don't remember exactly) to get a script from a doctor, and then the cost of the medication: insulin is expensive but nowhere as bad as it would be in the US. I'd recommend taking enough with you for the whole trip - 3 months isn't that long.