First, there are a number of scam listings, by people who pretend to have a room available, to scam the deposit or bond from you. For example, a "person" called Heather Buffington Nicole pretends to have a room for rent in Cathedral Place: 41 gotha street, brisbane. This is just a scam. See, for example, this blog.
Second, there can be disputes between flatmates regarding rent and bonds, and they get very messy. Often, one flatmate is held responsible for the conduct of another flatmate.