Many spectacular walks allow you to appreciate this rugged landscape, but before you tackle them, stop by the DOC Nelson Lakes Visitor Centre for maps, track/weather updates and to pay your hut or camping fees.
There are two fantastic day hikes to be had. The five-hour Mt Robert Circuit Track starts at Mt Robert car park – a short drive away from St Arnaud, serviced by Nelson Lakes Shuttles – and circumnavigates the mountain. The optional side trip along Robert Ridge offers staggering views into the heart of the national park. Alternatively, the St Arnaud Range Track (five hours return), on the east side of the lake, climbs steadily to the ridge line adjacent to Parachute Rocks. Both tracks are strenuous, but reward with jaw-dropping vistas of glaciated valleys, arête peaks and Lake Rotoiti. Only attempt these tramps in fine weather. At other times they are both pointless (no views) and dangerous.
There are also plenty of shorter (and flatter) walks from Lake Rotoiti's Kerr Bay and the road end at Lake Rotoroa. These and the longer day tramps are described in DOC's Walks in Nelson Lakes National Park pamphlet ($2).
The fit and well equipped can embark upon longer hikes such as the Lake Angelus Track. This magnificent two-to-three-day tramp follows Robert Ridge to Lake Angelus, where you can stay at the fine Angelus Hut (adult/child $20/10, bookings essential late November to April; backcountry pass/tickets valid the rest of the year) for a night or two before returning to St Arnaud via one of three routes. Pick up or download DOC's Angelus Hut Tracks & Routes pamphlet ($2) for more details. And if you've heard about Blue Lake, seek advice from the visitor centre before you even so much as contemplate it.