Hi Ian - nice pick-up. This is a Royal Tigertail, a relatively uncommon species in the ACT. I've picked them up at three locations in ACT so far (Pierces Creek, Callum Brae and National Arboretum), and I know of a record from the big dam at Mulligans Flat as well. They always seem to occur at (be restricted to) isolated clay-based 'farm' dams.
Yes, thanks. I could see it was probably a tigertail, but I don't know them well. A beautifully named and looking species. You clearly have to be lucky to see them perching in that apparently characteristic pose to be able to get a good photo. I see Roger Farrow has a fine photo of one in his 'Insects of SE Australia' book, p. XIV.