Sutherland Falls in Fiordland is the highest of all the New Zealand waterfalls at a staggering 580m. It is however one of the more difficult waterfalls to access, with the only option being a 4 day hike (Milford Track) or by scenic flight or helicopter. The water from Sutherland Falls drops from Lake Quill above, and falls down three tiers before reaching the bottom.
Glenorchy Air mentioned in my guide does scenic flights that include Sutherland Falls if the 53km Milford Track is not an option for you.