Whilst this particular example by the Benson Institute is situated in a Northern Hemisphere cold climate region, the same principle should apply in Australian conditions. Obviously the aim would be to provide relief from the heat rather than cold by tapping into the thermal mass of soil.
In it’s simplest form it is a rectangular hole in the ground 1.5 – 2 meters deep and covered by plastic or ideally plexi-glass greenhouse sheeting. A rammed earth wall (or dry-stone if you want to get fancy) at the back of the construction and a lower wall at the front provide the angle for the clear. The roof seals the underground greenhouse providing an insulating airspace and allowing the sun to warm the space providing a stable environment for plant growth.
Here is a follow-up video from the original author showing a subsequent year
Benson Institute Manuals: http://www.bensoninstitute.org/Publication/Manuals/
Pure Energy Services: http://peswiki.com/index.php/Directory:Walipini_Underground_Greenhouses