In the Vic Pioneers Index, there is a Robert Dunn death in 1871 (Reg no. 2019) aged 50, born TYRO but lists parents as unknown. I looked in the Federation and Edwardian indexes up to 1913 but neither list any deaths for a Robert Dunn with the parents you have listed.
Also in the pioneers index are children born to a Robert Dunn and Catherine McKinley, all born at Ballarat. There is Margaret (1857), Catherine (1858), Lilias (1861), John (1863), William (1867) and Mary (1870). I cannot find a marriage between these two, so they may not be related to the Robert Dunn you are searching for, but just in case I have given you the info.