As per Dave Noble and minz, a flex fare is entitled to a full refund. In cash to the original form of payment. BUT, in some (all?) cases, the online way to do this tries to guide you to a credit voucher, and to actually get a full cash refund requires you to call them. This is a sick and downright rude way to do things, but this is what it is. I recently cancelled a flex fare and was led on this merry trip. But when I did call them, the full cash return was happily achieved, even though, due to Qantas mysterious workings, it will allegedly take a couple of months.