It could be either Acacia cultriformis (which has one longitudinal nerve in the phyllodes) (feral in Canberra), or Acacia pravissima (which has two longitudinal nerves in the phyllodes) (probably feral in suburban Canberra).
I have grown both A cultriformis and A pravissima in my garden for 20 years and this plant was definitely A. pravissima. It is growing well away from the suburbs that fringe Farrer Ridge, but it could have come from someone's garden. There are about 3 occurrences of A. pravissima on Farrer Ridge, but I have not seen the other 2 recently.