I'm not far in, but it seems it may be something you might find interesting. It's documentary style (not found footage), about Something in the abandoned old train tunnels under Sydney.

I have a problem with Australian "horror" films. Many of them, of the slasher variety, actually manage to make me angry, in their brutality, and this is coming from me. Wolf Creek, for example.

But, this sounds different, and I'll check it out.