in the power store database, edit the wa_settings table using PHPMyAdmin or another MySQL GUI Tool.
find the record where the Name column is "Currency"
and change the Options column from:
Canadian Dollar ($):CAD;U.S. Dollar ($):USD;Euro (&euro):EUR; Great Britain Pound (£):GBP; Australian Dollar($):AUD
to:
Canadian Dollar ($):CAD;U.S. Dollar ($):USD;Euro (&euro):EUR; Great Britain Pound (£):GBP; Australian Dollar($):AUD; New Zealand Dollar($):NZD
then in the settings panel, you will be able to select new Zealand Dolor as the stores currency.