The crown range if I remember correctly is the highest road in NZ. It's just above queenstown. I went to Dunedin too (and cycled up the steepest road in the world, which is there). The central Otago rail trail is a must do, especially on a bike! I don't remember any particularly bad roads, and there was certainly much less traffic than in the UK, especially once you get away from the coastal area.
Sounds great. I'm taking my hardtail MTB with me rather than building up a tourer; I'm hoping to explore all the gravel roads and old mining tracks that don't exist in Europe... from Dunedin you can take the train (the single train in the whole of the south island!) out to Pukerangi/Middlemarch which is the start of the Rail Trail and various other trails, too.
Did you camp wild or in campsites? I find the law quite confusing... as far as I can tell it's okay to camp on government land so long as there's not a sign saying you can't... as well as private land if you have permission, obviously. Did you stay in any of those free/cheap unmanned huts?