Have you done any fly fishing before? Just learning to cast will be your first challenge.
I think you could use an 8wt for trout but a 6wt would be more suitable and a lot more fun. What sort of saltwater fishing are you thinking of? I've caught Barra up to 62cm and tarpon over 50cm on my 6wt, though you are restricted on size of fly with the lighter stuff.
I fished Tasmania last year (Twin Lakes) and caught trout up to 4kg on the 6wt. I didn't feel the need for anything heavier.
Are you thinking of using a guide in NZ? If so they would probably be able to lend you a suitable rod.