The single handed double spey is a fantastic, relatively simple cast to learn. It is probably one of the most useful techniques for fishing the Tongariro. There is basically no making around. The time wasted from the end of your drift to the time when your line is back in the water fishing is absolutely minimal.
The approach is to strip in a bit of line if necessary just like when you begin to aerial cast in order to make it manageable. Sweep the rod around your body upstream and then sweep back around your body downstream slipping a little line if you can and sweeping around back and up a little to create as tight a possible D loop behind you. When the rod loads up and while you still have an anchor of a little line and your flies in the water - cast - hauling at the some time if you can.
If you are initially unable to view the movie (MPG) file to the right of this (it may take a few seconds or even a couple of minutes to download of you have a slow connection so bare with it a little) then click here. This will hopefully open the application that your computer uses by default to view the movie - after clicking, you may have to check the other open programs as it may not come automatically to the surface.