Drive west along SH63 from St Arnaud and turn left on to West Bay Road. Follow it for 5km, crossing the Buller River and climbing to the upper Mt Robert car park. Walking time from here to Angelus Hut (36 bunks, serviced) is 5-8 hours. The NZ Mountain Safety Council has produced a video on the route which can be found here: https://www.youtube.com/watch?v=sg5BAkJwvMg
The route to Angelus Hut along Robert Ridge is a superb ski tour but beware of avalanches on the two sidles. A more sheltered access is via the track to Speargrass Hut (12 bunks, serviced) and then a poled route to Robert Ridge, or via the Cascade Track up Hukere Stream (beware of ice here in winter). Allow 5-7 hours for either option (less if you take the water taxi for the latter).
From the car park the Pinchgut Track climbs to Bushedge Shelter at the bushline, just north-west of Mt Robert (1421m). For a good round trip continue to Relax Shelter then head east to Bushline Hut (14 bunks, serviced) and descend Paddys Track to the car park. The main ridge track continues up a tussock spur to Second Basin, overlooking the old skifield. Follow the rock and scree ridge over Flagtop (1690m) and above Third Basin to a notch below Julius Summit (1794m). To continue south along the Robert Ridge, descend scree to a hanging basin on the west side, sidle under Julius Summit and regain the ridge. Follow the ridge above Fourth Basin to a south-east spur at its southern end. Peaks 1885m and 1877m on this spur are steep scrambles. The track to Lake Angelus sidles the western end of this spur, down to a saddle at the head of Speargrass Creek. Climb to the ridge above, on the eastern side of the lake, and descend scree to Angelus Hut.