The 5261 postcode covers Cooke Plains, Coomandook, Yumali, Ki Ki and Culburra. I've chosen Ki Ki to place a quick letterbox to break your trip on Highway 8 whether traveling northwest or southeast. Park near the listed coordinates and you can walk to the endpoint.
PLEASE NOTE THAT THE END POINT IS NOT FAR FROM AN UNFENCED ACTIVE TRAIN LINE. KEEP AWAY FROM THE TRACKS AND WATCH YOUR CHILDREN AND PETS.
At the listed coordinates there is a real letterbox, a mechanic's garage, and 30 post boxes. An unusual grouping of services, but that's the way it works in small towns.
Let ABC be the number of the top right box.
Let DEF be the number of the bottom left box.
The cache can be found at S 35º 34.(F - D)(A + B)(C - E) E 139º 47.(C - B)DA Checksum of decimal place digits is 25.
Replace the cache with some natural camo cover, and remember that the stamp stays in place in a letterbox cache.
Have fun, stretch your legs and have a safe onward journey.