The Cache
The cache is located just off the path. You are going to need to careful with your timing, as walkers exiting the tunnel will appear without warning.
The Conservation Park
The Mount George Recreation Zone is in Mount George Conservation Park. This park runs along the northern edge of the South Eastern Freeway between the Stirling Golf Club and Bridgewater. You will have to navigate a few delightfully winding roads to get to the Rec Zone. Alternatively, you can park at the end of Arbury Park Road and walk along the Heysen Trail through the tunnel.
Viewing the map on your computer is not as interesting as using your phone. Just north of the listed coordinates you will find Tunnel Doof shown on the map. This appears to be right where the rotunda is located in the recreation area. Let me know in your log what you think Tunnel Doof is.
Enjoy a walk around this wonderful Conservation Park. Cox Creek meanders through the park, with some lovely lakes, wetlands and bridges to enhance your walking experience.