Aaron with aged specimens it is difficult to be certain. It looks a bit like a Gymnopilus junonius because of colour, and robust nature of it's form. The inversion of the cap and fragmentation of gills most probably result from the specimen experiencing some adverse conditions!! Effective photo though.
Like Pam I'll go for Gymnopilus junonius as highly plausible. In Canberra if you have something with such a combination of size, colour, robustness and clustered growth that species is the most likely candidate.