From what I read, it might not be possible to change your localization information on the fly. I also read, that Opera had serious security issues (3rd party applications were allowed to set your default search engine...), which lead to the curren situation, that some - former available features - are no longer supported or not yet implemented in current versions of the browser.
Maybe a complete uninstallation and re-installation of the browser helps ( so that NZ becomes your default) , otherwise you can still press Alt+P in Opera and define a Custom Search Engine for New Zealand (use this URL:
https://www.google.co.nz/?gws_rd=ssl#q=%s), in that case you'll have to use a prefix in your searches to use that Custom Search Engine, as you can currently not set Custom Search Engines as default (or maybe I just didn't find the spot to do)
If another browser is an option for you, you should might try ;-))
Good luck