Not an official track anymore. In former time there was a track from Lake Manapouri to Doubtful Sound. Now you can take a boat over Lake Manapouri and then take bus to the sound. However, part of the old track is still there, and the rest can be walked on the road (ca. 20km) via Wilmot Pass (673m). This is easily done in 4-6 hours and is more rewarding than the bus trip. You can book a place in the youth hostel at Doubtful Sound and make several day trips from there. The wardens there are extremely friendly and supplied me with freshly catched fish! The 3h cruise on the Doubtful Sound is highly recommended. After the tourist busses have left (5pm) you are almost alone! The atmosphere here is much better than at Milford Sound with heaps of wildlife around (including the infamous sandflies).
Hall Arm of Doubtful Sound.
View of Doubtful Sound from Wilmot Pass (673m).
Doubtful Sound viewed from the Tasmanian sea.
Try to ignore the cruise boat and have a look at the amazingly steep valleys under Commander Peak (1275m) in the backgound.