I was concerned about this sighting and went back 03/06 and can confirm all the leaves are reddish purple beneath, same colour as the other Acianthus exsertus observed at ANBG Southern Annexe earlier on each day. DerekC
I assume this is a known sight? I am intending to again return and see if I can get a better photo of flower open but if you are correct this will be obvious too. I had considered, because of the poor flowering, if it could have been 'A. collinus' but the stem of collinus does not appear to be purplish (ACT Field Guide) as in my photo.
Yes there are several known colonies of A. exsertus in the Bushland Annexe. I considered A. collinus which are just starting to appear but I have only seen one plant with a flower to date. Also the stem tends to be more greenish and the dorsal sepal broader than what your image appears to show.