A species of Calvatia. A couple of species, at least, are likely to turn up in grasslands or sparsely wooded areas (such as shown in your photos). Eventually the outer skin cracks, breaks and falls off in pieces to expose the powdery spores. At least one of the largish Calvatia species found around Canberra has a dull violet to brownish-purple spore mass. Most of the puffball-like fungi have spores some shade of brown, with no hint of violet or purple.