A thank you to all those who responded on and off-list to my original query re: the relative possibility of a White-winged Triller or Rufous Songlark in a Canberra garden.
Many helpful comments. My inclination now, having considered suggestions and a range of photos, is that the bird was more than likely neither of these but rather, an Australian Reed-Warbler. I did have a definite, though fleeting, impression of a small crest,
which points in this direction.
While I'm pretty unfamiliar with either the Triller or Songlark, I have seen many a Reed-warbler. It's interesting how spotting a relatively familiar bird 'out of context' (first time in my own, or anyone's, backyard!) can throw you off. But one result is
that I'm far more prepared to positively identify my next actual Triller or Songlark...