I don't get many hits from New Zealand, but I see quite a few Aussies. Given that I'm starting to plan a 4-6 week trip to both countries, it seems a good place to ask for advice.
The idea is to cram what I can into just one trip. I very much want to visit your countries, but I very much want to avoid 18 hour flights even more. This is therefore likely to be my only trip to your part of the world, so I want to prioritize and plan.
The NZ south island and Australian east coast are happening, of course. Beyond that, what would you tell your American cousin he shouldn't miss?