Possibly Blechnum patersonii, but I've sought a second opinion. This area might be better covered in the Coastal Wilderness website, as it's in a very different climatic type than the Canberra region. See https://www.inaturalist.org/projects/atlas-of-life-in-the-coastal-wilderness
G. billardierei often grows on trunks (tree ferns etc) so this tends to confirm the ID, but may be G. meridionalis, depending how long the fronds are. How long were the longer fronds?
Based on Jones & Clemesha, it looks like the ID should be Grammitis meridionalis, but CSIRO's much more recent Flora of Australia says the latter is just a small habitat version of G. billardierei. So the ID stands.