Hi Kim, I'm just going off Reiner Richter's website again (magnificent photography...), it mentions that a 'tentative' ID for Hemicordulia tau is its yellow pterostigma on its wings, while Hemicordulia australiae has black. It also mentions other more definitive ID markers but I don't think my photo provides sufficient information. http://photos.rnr.id.au/Data/Animals/Insects/Odonata/Hemicordulia%20australiae.html