JVR - If you can could you please go back try to find it and take photos of the flowerhead's underside and perhaps from the side? It looks like the Fireweed S. madagascariensis a highly invasive species which we desperately want to keep out. We found a couple at the lower west slope see for instance Senecio madagascariensis (Madagascan Fireweed, Fireweed) and we assume seeds had been blown into Mt Majura nature reserve from Gungahlin.
I would just pull it out and bag it, in my mind there is no doubt this is madagascan fireweed. Mt Majura doesn't have a locally occuring look alike species. Senecio pinnatifolius var. alpinus is the the one usually confused with madagascan fireweed, but this doesn't occur in Canberra as it grows in above 1200m.