Don’t worry, the insulating glass units (IGU) are easily replaced locally.
The glass in our windows is secured by fasteners behind the window trim. Simply remove the window trim from the window and replace the IGU with dual paned glass sourced locally, replace the fasteners, and trim. If you would like to replace your glass with similar glass, remember to ask for low-E coatings and argon fill because these are definitely not standard in New Zealand.
For more complete step by step illustrated instructions, please see this document.