How much study gap is acceptable in New Zealand?
It sounds like you have still being doing research and publishing since your masters. If you have still been keeping your skills fresh, that means you haven't really had a gap, regardless of whether or not you were registered as a student. Just explain this when applying and your gap will not be a concern. Your recommendation letters and publications can attest to that fact.
Age should not be a concern from the university's perspective. Although, a prospective supervisor may wonder about your perspective -- why do you want to do a PhD now? why not earlier? If you can give a good answer about your aspirations, then that will allay any concerns.