I am assuming this is actually a below-ground type, as I didn't actually go digging to see if it concrete all the way down. It seems fair to assume there is more below than above, as there was a bit of a hill it is set into, not quite visible in the picture.
I was looking for a particularly devious cache adjacent to the picture, but never located it. Going back that way tomorrow, so will try again.
Located in Tusmore (a posher part of Eastern Adelaide suburbs), this is a very secret little part of the world. Easy access apart from the long grass, no fences or anything.