Australian Stock Saddles are built somewhat like English saddles (if they are the traditional kind) with wool (or other material) filled panels. These provide all the padding you need, unlike the western saddles, where you have to use thick saddle pads to cushion the tree.
So really what you're looking for is a saddle pad that keeps your saddle clean, and the horse dry and cool. For this wool or at least wool lined is often a really good option. Alternatively, you can use a standard english pad. Normally dressage pads are good (they have straight fronts rather than curved ones) and you might need it in an extra large size to accommodate the saddle.