Route from Otehake River to Koropuku Hut via Koropuku Gorge
As you travel along the Otehake track, the junction of the Koropuku with the Otehake will become visible. When you figure you are above the junction, leave the track and head down towards the junction, As you approach it, you can fine tune your direction to come out opposite the junction. You will have to find the only spot on the bank where you can easily descend down to the true left of the Otehake. The sound of the rapid downstream is quite intimidating. It takes about half an hour to arrive here and cross the river from leaving the track.
From where you managed to drop down, go upstream on water smoothed greywacke walls and boulders of the gorge to an obvious ford about 10m or so upstream from memory. It is obvious where the ford is. It shouldn’t be attempted if it is more than waist deep if there is any current. The times I have forded it recently, it has been knee high with no current. Be aware however that the nature of a ford can change.
After fording the Otehake, travel down to the junction on the true right of the Otehake. I think this is about 20 metres but it is all obvious. Cross the Korupuku to the true right and then shortly after this, cross back to the true left. You will encounter a 3m or so rock scramble which has good holds, footings and grip but is slightly exposed. Further up, there is an obstacle of rock but towards the true right there is a diagonal sloping dry route. However this may now have water draining over it as it did the first and second times we went up the gorge.
Travel is straight forward beyond the Tawhana stream junction until just before the Big Gully. Here there’s a waterfall so you’ll need to ascend about 5 meters up greasy mossy slopes with minimal vegetation on the true right of Koropuku Stream, sidle briefly then drop back down to the stream. Travel upstream beyond the Big Gully/Koropuku Ck junction a short way (10 metres) then cross where it looks the best. Here there are large boulders and a bit of a beach from memory. To get to this point from the Otehake/Koropuku junction would probably take between one and two hours. There is a visible cairn built a rocky outcrop on the  true right of the Big Gully stream.
If you want to check out the spectacular Koropuku Waterfalls, travel about 15 minutes upstream. Travel is straightforward to the falls.
At the junction with Big Gully (NZGD 49: E2406058, N5819487 or NZGD2000: E1496086, N5257880) you’ll find a short track cut through scrub that brings you to the true right in the gully. Travel up Big Gully, and on the true right after about 10 minutes height gain, you’ll find a cairn indicating the traversing slightly downvalley route which takes you to the steeper part of the ascent. This cairn is just upstream of a recent rock slip. The route has various markers remaining hopefully which consist of red insulation tape and white plastic strips from supermarket bags.
The steeper ascent out of the gully is about 6 meters using a 15m orange rope suspended from above and small trees and saplings to assist you. It is slightly exposed. Please take care not to destroy any of the foot and handholds. It is easiest on the right side facing uphill. I don’t think there is any other way up out of the true right of the Big Gully. You will find more red insulation tape as you exit the gully onto the spur. It takes about 20 minutes to reach here from the Big Gully/Koropuku Stream junction.
From here on, there are no markers. Just sidle upstream, maintaining the same height and by trial and error you’ll find the easiest line to sidle to opposite the junction of the Koropuku Access gully. You’ll cross 2 streams enroute to this point. One is dry and wide from a recent heavy scouring flow. When you are opposite the access gully, after approximately one hour, cross to the true right of the Koropuku and sidle half an hour upvalley to the hut as per other notes on this site, or Remote Huts website or The Tramping Report.
Total time from leaving the Otehake track to arriving at Koropuku Hut is about 4 to 5 hours. Total time for travel from Aickens to the hut via this route would range from 8 hours to 14 hours.