UHD: "...we assumed our roof was a cheap iron product with an inferior paint finish." This isn't NZ Steel's fault and if the paint loss was that serious it should have been sorted out years ago. As soon as you moved in, really. Home maintenance is no joke. It is certainly unfortunate that this has happened and also falls just outside of the warranty but you need to acknowledge your maintenance failure and move on IMO.
I think maintenance failure is a bit harsh. We've previously had the roof checked for leaks, replaced loose nails etc. At the time we were advised that there was no urgency for immediate painting as the iron was in ok condition, it's just the paint finish that was failing - which I am now taking care of. As I said in my opening post, it was only during the pre-paint inspection by our roofer that we learned it was Colorsteel.