I guess that is one way of looking at it, but I would have said that the Murray-Darling starts in the headwaters of the Condamine R, in the mountains east of Killarney in SE Qld.
I'll agree with you on that. Looking at the charts, the water off the western side of Mt Roberts on the Border at 4531 ft has further to fall, than the 3759 from Mt Kiangakow in the Bunyas, and further to run, with a big catchment for the Condamine. My father would have been more familiar with that area, having grown up at Cambooya. Just that as the most north-easterly point for the south-westerly flowing system, for me, the catchment starts on the western side of the Bunyas, or Durong.