gzt:

when they are being replaced every 1-2 years due to changes in the eyes.

Eek! I'm just starting out with glasses. Is that common?

Thankfully not for everyone!, we went for ~5 years not needing any new pairs, but then the last 6 years or so it's been a reasonably regular change. It's worth getting the regular checks, some people wonder why they are getting headaches etc... and it can be that your vision has been changing over time. 

It's another case of NZ being on the far end of a supply chain with a low population, just compare the deals between specsavers Aus and NZ, the lenses are all shipped from Aus for NZ orders, but NZ pays significantly more even for single vision lenses.