Depending on exactly where it is in Namadgi NP, highly likely to be soft tree fern - if so, will have a trunk under all of those fronds. Certainly not a Blechnum (which you can see on right side of photo) and too big for Polystichum.
It is definitely a Dicksonia antarctica. The numerous dead fronds hanging down beside the trunk are characteristic, as are the convex frond surfaces. Cyathea australis has concave fronds, as viewed from above. This means of discriminating is not listed in the keys, as they are almost always derived from pressed specimens. There must be a large reservoir of C. australis somewhere in the ACT mountains, but I have yet to find it. D. antarctica is very common in ACT fern gullies.
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