The underside is white when the fruiting body is young but becomes browner after release of the spores, as here.
This a woody fungus which has pores rather than gills (known as a polypore). It lives for many years and has growth rings like a tree. The shrivelled older parts of the fungus can be seen above the newer layers, as shown here. Ganoderma means shining skin although it doesn't look very shiny here. (Australe means southern, not Australian.)
The annual growth rings show the fungus is getting larger each year.
Here is a close up view of the underside showing the pores and where some of the cinnamon-coloured spores have fallen on to the bark of the tree.