I didn't even know it had been done. The locomotives and rolling stock are available as far as I know, but not sure about the route.
I wrote a few books and publications about Walhalla many years ago including Walhalla Today and the Switzerland of Australia (both co-authored) and the Walhalla Chronicle newspaper (still being sold) and ran a Steamrail trip there around 1979. It was a huge success. I travelled on the narrow-gauge train there for the first time last year.