Probably a Yucca or one of the related species Dasylirion, Furcraea, Hesperaloe, Hesperoyucca, or Nolina. The Wikipedia link: https://en.wikipedia.org/wiki/Yucca It will need someone with access to a North American Flora to ID it properly.
Michael I'm pretty sure that this is Spanish Bayonet as it is a common planting in ACT gardens. I have also ID Yucca filamentosa as a naturalised plant in Canberra, but it has white hair like curly filaments on the leaf.
This looks more like a Cordyline. For Yucca aloifolia see sighting http://canberranaturemap.org/Community/Sightings/Details/34310 on Isaacs Ridge. I think the plants at the top end of Long Gully Road have now been removed but there is one in the new offset and possibly others part way down the hill.
I have had a look at the Plantnet key to families. It keys to Asteliaceae (the new family for Cordyline) if the perianth segments ('petals') are less than 2 cm long and the leaves are not succulent or spinescent, and not Asteliaceae if not. Then only two leads later to Agavaceae (the family for Yucca) if the perianth segments are more than 2cm long and the leaves succulent or spinescent. Could you advise, Michael B?
The petals are less than 2 cm long. The tips of the leaves are pointy, but soft, not sharp to to the touch. Also the leaves are sort of leathery and not succulent.
I located this plant today to collect a herbarium specimen; the three stems were all dead; it may shoot back from the base. I didn't see any plants near the footbridge.
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