The well-maintained track continues downriver from Upper Matakuhia hut, despite warnings in the hut book to the contrary. It continues benched, and recently cut (2014), crossing to true right, then back to true left, until finally, 1km before the track junction, all maintenance ceases. Seeing how it was prior to their work makes me only appreciate more the work the local hunters have done to open it up: huge windfall ahs obliterated the track. Tall scrub and fern has obscured it. Slips have destroyed it. Following it beyond here is not even worth considering. The best option is drop to the creek and follow that: easy going until a gorge just above the track junction. Here markers point you back up the true left bank, and a faint but followable route sidles above the gorge to where a large signpost marks the junction.
Note: Described in the reverse direction to your journey
Heading upriver from the track jhunction, the track is uncut but marked with permolat / tape for 100m or so, before it drops back to the river. A sidle track did exist up the next section, but it is overgrown, unmaintained and unusable (though it is slowly being recut, so things may have improved).
The riverbed provides reasonable travel for the next 1km - with windfall providing the only real obstacle. The stream is small - ankle to knee deep in normal flows, but could, conceivably flood. Passing the major sidecreek on the west, a sidle track soon appears some 20m above the river on the eastern bank. There is no indication of where this starts though, so you may just continue up the creek a further 300m to where the map shows the track crossing, and pick up the track as it crosses to the western bank.
From here the track has been recut by local hunters (2014) and is well maintained. There has been a lot of windfall, and so you often still have to climb over fallen trunks on chainsaw-cut steps. Passing a waterfall in the creek, the track returns to the eastern bank, and soon the valley widens and we drop to flats on the valley floor. Upper Matakuhia Hut is on the eastern bank, about 2km above the last river crossing.