From Potts Hut, head 100m downriver, crossing the 1st sidecreek. This is ankle deep in normal flows, but can clearly flood in heavy rainfall.
Scramble onto the terrace just across the sidecreek and pick up the old 4WD track, which zigzags its way up a series of terraces before sidling back into the sidecreek you just crossed, which it follows east into a hanging valley above. A small tarn lies just beyond, next to the track, supposedly with viable camp-spots, though I saw none.
300m beyond the tarn, the track swings sharply right (south) and starts to zigzag steeply up onto the Dog Range to the south - a decent climb. Once on the top the track follows the ridgeline ESE, descending in a series of steps for approx 5km before finally swinging south to reach a broad tussock saddle where a signpost marks the turnoff to Mystery Lake
Note: Described in the reverse direction to your journey
From the Mystery Lake turnoff, the 4WD track to Potts Hut swings north and starts to climb steadily to reach the tops of the Dog Range. A series of stepped climbs west follow, with flat saddles in between before the highpoint at pt1546 is reached.
200m west of the highpoint the track forks. Take the right-hand (northern) fork sidling then dropping towards the valley to the north in a series of zigzags. This is where 4WD and ATV access really end (unless you feel like tussock bashing on steep slopes beside the track) with the track scoured out by water beyond vehicular use.
At the base of the slope the track swings NW downstream following the southern bank of a small marshy creek, soon reaching an unnamed lake. There are, supposedly, viable camping spots here, but in mist and rain none were obvious when I passed through.
The hanging valley ends suddenly, and the now very rough and narrow former vehicle track follows the creek down before sidling onto the southern (left-hand) face and dropping down a series of river terraces to the main valley below. The last 10m to the creek is steep and crumbly and seems to stop the last of the ATVs, though one local motorcyclist (at least) has mastered it.
Potts Hut is visible 100m upriver on the eastern bank, across a small sidecreek. This is ankle deep in normal flows but could clearly flood in extreme weather.