These are plants that grow in fresh water, such as in rivers, lakes and swamps. But this category excludes Rushes & Sedges, Lilies & Irises, Daisies and Ferns which have their own categories.
Any chance of adding a photo that clearly shows the fruiting head ? There are some in the second photo, but not clear enough to show the shape of the beak (little projection of each achene).
I don't think it is rare in the ACT; see the large number of sightings in CNM. It has been grossly under-recorded by vegetation survey and mapping. its under recorded because most veg survey doesn't consider the habitats where this species occurs.