Cassinia arcuata, Sifton-bush or Chinese-shrub. This species naturally occurs in low densities right throughout the region and also in many habitats ranging from natural grassland through to grassy woodland, and dry forest communities. It has a tendency to become invasive only following a disturbance, or following cessation of continuing disturbance, for example, set-stocking grazing.
Cassinia arcuata is seemingly the name this plant used to go by, a few relevant comments are associated with this sighting https://canberra.naturemapr.org/Community/Sightings/Details/3404473 Cassinia sifton seems to be the current name for what we have in the ACT.
The Commonwealth of Australia Gazette 26 September 1935 listed Cassinia arcuata as a weed pest. Strangely this was proclaimed under the Quarantine Act prohibiting its introduction into Australia if there were more than 20 seeds per ounce when mixed with other seeds.
I’ve done a little research and have found “Cassinia sifton” according to one authority (https://vicflora.rbg.vic.gov.au/flora/taxon/b63da76d-f3d0-44b0-bf60-803c7d9de56c). However, NSW PlantNet (http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Cassinia~arcuata) and Australian Virtual Herbarium (https://avh.ala.org.au/occurrences/search?taxa=Cassinia+arcuata#tab_mapView) with links to ALA, still both have it as Cassinia aculeata.