I spied these rocks while seeking another cache on the mountain. This should be a fairly easy find, although to reach GZ there's a short steep scramble off the track which may be inadvisable in the wet. There were squillions of mosquitoes when I placed the hide, so some insect repellent might also be a good idea in the warmer months.
It's not necessary to pass through the tunnel to reach GZ if you don't want to, as there's an easy walk around the outside. Since the area is likely to be visited by muggles, please make sure the cache is inconspicuous when rehiding. The cache isn't in the tunnel as I couldn't find anywhere in there where I could reasonably conceal it from curious eyes, instead it's about 8 metres south.
Access is along the walking track from the end of Mullbong Road. Take the track on the left, not the fire trail! The track leads to the top of the mountain, so you can continue on to the other caches up there if you wish.
The container is a 200ml Sistema with not much room for swaps. A pencil is included, but BYO pen just to be sure - it's a fair hike back to the car to get one!
Congratulations to GEO247 for FTF. Enjoy!