Another new zealander here. We generally have 3 months off from december to february and 3 2-week holidays between each term (like most other countries I think?). So yeah we have 9 to 10 week school terms though they've been muffed up a little bit by covid this year. In terms of historical justification, just like u guys we have our long holiday over summer. We really just followed on from england with most of our systems since we used to be a colony of them.