The issue with length of time is to do with taxes ,if you have owned the glider for a year then the glider is 'personal baggage' and not taxable.Even if the IRD deem tax is payable (12.5%) When the glider is re-exported this can be claimed back. Re export is simple paperwork exercise, The issue with experimental is to do with service records and history acceptable to the Civil Aviation authorities to allow your glider to fly in NZ. Since you are going to the Omarama area I would email one of the operations there and get there input as they have some experience with exactly what you are proposing.
www.GlideOmarama.com gary