Aristida vagans has been on the Mt Majura flora list since 2006 - compiled from Ingwerson F, Evans O, Griffiths B (1974) Vegetation of the Ainslie-Majura Reserve. Conservation Series No 2. AGPS Canberra. However it does not appear in the 2019 Census of ACT plants. ALA has old records from Black Mountain and quite recent ones from Queanbeyan, and one from the Hume area in the ACT.
Since Grassman thinks it could be something interesting, I will endeavour to look in the vicinity of this sighting this summer and see what I can find.