I've been to NZ 8 times so far with another trip on the map for Feb 05. I've always used green or tan colored lines. Most of the time, in the situations I fish, if the fish sees your flyline (no matter the color), you are hosed. Long leaders are all the fish should see. I'd concentrate on the casting characteristics of the line and if possible, get a muted color of some sort just in case.
There are 2 main casting characteristics to consider if you are talking about the south island.
1) Wind. It is a windy bugger down there much of the time. In some valleys, it would be impossible to exaggerate the wind power. Get something you can drive into the wind if you need to - or at least have that sort of line available.
2) Close Quarters Much of the time you are fishing with restricted casting room.
I've always had 4-6wt 9' rods, but this year I'll try a sage 6wt 10' xp. Mostly for increased roll casting ability over the 9'. I love my 8100xp for steelhead. I've tried the Cortland "spring creek" lines, but I don't care for them too much. I like the Cortland rocket taper line (and SA steelhead taper) for its roll casting, but it requires you have a fair bit of line out to load the rod so it isn't that great in the wind.
This year, for my main rod, I'll have both a shorter (wind) and longer (roll casting) belly line.
Think wind and close quarters.