Thanks Michael - this is not on our rare plant list but maybe it should be, There is only one ACT record of this plant in the Atlas of Living Australia a 2006 recording by Isobel Crawford also on Lake Burley Griffin at Acton. It is not listed on the ACT census of vascular plants and is mainly a coastal species.
If you look at the other sighting, you will see that it can be quite dominating and copes very well with mowing. There was a dense patch of it maybe 30 metres across. See http://canberranaturemap.org/Community/Sightings/Details/3342917