This a rust - not sure which one - there is native rust (Puccinia morrisoni) which affects pelargoniums and another rust (P. pelargonii-zonalis) which is a serious diseases of some cultivated species. I am not sure whether to put this sighting under the host (Geranium solanderi) or the rust!
Sorry Ruth one needs to look at the associated fungi with a microscope. John Walker in his paper "The rusts of Geraniaceae in Australia" (Polish Botanical Journal 55(2): 315–334, 2010) listed two species on Geranium (Puccinia geranii-pilosi and Uromyces geranii), two species on Pelargonium (Puccinia pelargonii-zonalis and Puccinia morrisonii) and the possibility of several other species yet to be recorded from Australian Geraniaceae.