The West Coast has only brown trout. Rainbows have never been introduced. The first time I traveled New Zealand I spent two months fishing and tramping the West Coast. In all that time I had only one fish under four pounds (although it should be pointed out that I only fished the back country).
The West Coast gets a very high annual rainfall, not too dissimilar to Fiordland. In an attempt at wit they call it the 'wet coast'.
I have fished both xxx and xxxxxx of this region quite extensively, however not very much in-between.
xxxxxxx River System
The best backcountry brown trout fishing in NZ comes at a premium: you have to walk in, and it's no picnic. Technically you could kayak down the river, but it's a grade 5, so you'd better know your stuff, and even then you've got a problem getting your boat to the river (you have to climb a mountain first). There are a number of ways into the river. The xxxxxxxxx track is one of them: it's an old gold mining trail. It says a lot about the middle reaches of the xxxxxxx that the gold prospectors chose to take this detour rather than face the xxxxxxx Gorge.
The xxxxxxxx River
The upper reaches of this river have good fishing for sighted trout. Lots of brown in crystal clear water. Some of the pools are difficult to access (I carry a short piece of rope around so that I can climb down to these fish). The middle reaches have good fishing especially around the xxxxxxxxxxx lakes. The Lower xxxxxxxx is just too big for my tastes, but has a number a good size kahawai in the estuary which you can catch on the fly.
For trout flies the rule tends to be the bigger the better.
There are some big eels in this river. (In NZ the freshwater eels grow to 50 pounds, live 100 years, are aggressive and the biggest I have seen has been about 35lbs).
This is a small river, and a disappointment. It is fished very hard by the helicopter boys (who haven't spent 2 days walking to get there), a good fly not to try is the Royal Wulff. If you arrive fairly late in the season you can be pretty sure that the fish will be educated and spooky. In order to catch them you will have to go small. I have spooked fish here just with the fly (comprehensive spooking, see the fly, turn around, leave the pool, swim passed you, gone downstream sort of spooking) and you just don't get this sort of treatment in NZ.
If you are going to fish this river it helps if there are two of you, one to spot and one to fish.
Some good fishing, but there's better downstream!
The xxxxxxx xxxx
With a name like that, you've just got to go and check it out. A truly great river, well worth making the effort to get to. There is a comfortable hut at the bottom. Excellent fishing upriver. And the further you go the better it gets. Big leaded nymphs and big humpies work well.
The xxxxxxxxx River has a few fish at the bottom and then becomes gorgy.
Of which there are two. Very creepy place, lots of dead tree stumps emerging from the lakes. But there is good fishing and some very large fish in parts. A float-tube would be useful, but it's unlikely that any sane person is going to walk one in. Watch out for some quicksand around the edges.
In order to get from the xxxxxxx xxxx to the xxxx River you are going to have to walk past these two lakes, which involves a couple of high bluffs. No track exists between these two rivers: you just kind of make it up, and the walk takes about ten hours. It's ten hours for ten miles hard walking. You will see some large fish in the xxxxxxx though. Not many people fish for these fish.
If you get to this river you will have to bring a tent and plenty of insect repellant... the sandflies are atrocious! Some of the best brown trout fishing you will find anywhere. This river is a Wilderness Zone and so you won't be bothered with the chopper brigade. Excellent fishing throughout the river. The lower reaches become gorgy whereas the upper river flattens out. You will see trophy fish in this river.
The xxxxxxxx River
Another river well known for it's gold. It's very gorgy for many miles, but then opens out into some spectacular fishing. Typical 5 lb browns with some larger ones thrown in. Dries and Nymphs as always. When the river is flowing low you will have to fish smaller flies (down to #16's). Both branches offer good fishing, but the South appears to be best.
Of interest I managed to get stuck on this river. It rained and it rained and I was stuck for ten days in one hut waiting for the river to drop.
The xxxxxxx River
The river is held in some high regard. Worth a tramp. Lots of mozzies though!
This river has some good dryfly fishing when the cicada is on. Middle reaches are good, upper reaches are best. There is a campground at the bottom. Seatrout (and/or slob trout) can be found around the estuary.