From the parking area, follow the tramping Lakeside Track which in roughly 1.5hrs brings you to a track junction. It is just 10 minutes walk to the base of the waterfall from here.
Return via the same route, total walking time is 3hrs. Alternatively, you can walk the whole Lakeside Track, which takes roughly 5hrs.
Best way to visit the waterfall is to combine the walk and a water-taxi: from Kerr Bay (Lake Rotoiti) take a water-taxi (as per 2019: Adults $20 Children $10) which stops at Whisky Falls. It is just 10 minutes’ walk from here. Return either by the same water-taxi, or take a 1.5-hrs’ walk via Lakeside Track back to Mt Robert Rd.