I'm prepared to speculate the rock on which the fern is growing is limestone. A. trichomanes almost always (but not always always) grows on limestone. Excellent find of quite an uncommon fern!
Plenty of it (at least before the 2003 fires) growing on the grey limestone surrounding the Cotter Cave, and AFAIK this is not yet in the Canberra Nature Map register.
Thanks speedsta - I have made you a trusted user which means that you can now go back and edit this record and use the google map feature to click on the location. Common Spleenwort has a restricted occurrence in the ACT area so good to have a new location.