Yep, you WANT to use a thin pad with an Aussie saddle. Like an English saddle, it has the padding actually attached to the saddle (a western saddle does not = why western pads can be so thick). If the saddle fits well, all the pad is meant to do is keep the saddle sweat/grime free.
If the saddle does not fit well, some fit issues can be helped via a thicker pad but if the fit issues involve it being too narrow, a thick pad will actually make that worse. Similarly if you pad up a really well fitting saddle, it ends up being like if you wore thick socks with well fitting shoes - sorta ouchy after a few hours!
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.