This section of the Great North Walk follows the old bullock dray track once used to haul timber from the cedar forests around Yarramalong across the ridge to Mangrove Creek for transport by water to Sydney. According to legend, this boulder would dine on unsuspecting bullockies, but with their passing it's now hungry, very hungry.
To find the cache you'll need to go inside the boulder's belly if you dare. You won't need a torch unless attempting it at night, but that's really not a good idea. The cache is a 100ml plastic container so will only hold the smallest of trackables. Please make sure it's concealed out of sight as I'm sure this rock is a muggle-magnet.
From the parking waypoint, follow the GNW arrows down the steep hill through the Desolation of Transgrid before turning to the right between the third and fourth towers and entering the beautiful glades of Bumble Hill. Along the way you might even find another cache (GC71QN9). If you can arrange a car shuffle, it's well worth doing the complete walk all the way down to Yarramalong (about another 3km past GZ).