Branchlets hairless. The branchlets in Acacia mearnsii are hairy. Not Acacia decurrens, where the lleaf staks continue down smoothly into the stem (decurrent).
I was going by the ridges on the stems, and knowing that Acacia decurrens was planted in this area around 1998. I will need to compare A. decurrens (sharp ridges) and A. parramattensis (rounded ridges) according to NPA Field Guide to Native Trees of the ACT so I don't get confused in future.
Thanks Betty - like Mike I have also been confusing Acacia parramattenisis with A. decurrens just going on the ridged stem, which I now see isn't enough. Look forward to your on-line key to make us all even better botanists