A. porosus and A. pallidicollis look quite similar in the Canberra area (and A. porosus looks quite different in other parts of its range). Probably the best way to distinguish them is to look at the pygidium and its patterns of setae. A. pallidicollis has all of the pygidium covered in short white setae, A. porosus has setae along the bottom margins and through the midline, but bare patches along the sides of the midline. The latter seems to be the case here, so this should be A. porosus.