There is no way for private cars to drive directly to Doubtful Sound (like you can do for Milford Sound), so Doubtful Sound Cruise, either full day or overnight, needs to be booked from Te Anau or Queenstown. The cruise consists of a bus tour from Te Anau/Queenstown up to Manapouri Lake, then a boat tour crossing the Manapouri Lake, then (optional) a guided tour to the underground power station, then again bus through the Wilmot Pass, and at the end you'll do 2 hours or overnight cruise by boat along the Doubtful Sound.

You'll see Helena Falls on the right hand side a couple of minutes before the bus arrives into the Doubtful Sound. As soon as you get a first view of the fiord, you'll see this waterfall dropping into the fiord. It is the first waterfall which you will see in Doubtful Sound.