As you probably suspect, his fantasy world is easier to maintian while you and your dd are the other side of the world.
Go back when you want to, to build your own life. Keep very low expectations about contact with your ex partner - that's my advice. Then if there is some, you will be pleasantly surprised. His parents sound great and I am so glad you have such good support from them.
I really think you should get him to pay some maintenance - go for it big time. This must be easier to do once you are in Australia, but worth persuing now. And you say you and your ex partner own a house. Any chance him giving you his half as part of a maintenance agreement if he really is skint?
You know your ex partner. Is he more afraid of the financial or the emotional commitment? What is his denial of you and your dd based on? Would you settle for one without the other - maintenance but no contact or visa versa? Would he find it easier to give you maintenace if he knew you would make no emotional demands on him? is he afraid that by giving you maintenance he has to take up all other fatherly duties? I'd imagine this is something you have to discuss with a legal person. You could write a letter to him, but if you state in writing what you want and what you are prepared to forgo, this might be used against you - I don't know.
Also, how sure are you that he really is living with his sister, sleeping on her floor? this sounds like a temporary arrangement. If his sister is prepared to lie for him, as you say, he might be anywhere. Would his parents necessarily know the truth, and if they did, would they tell you? they might be unsure about his true wherebouts, not want to stir things or simply want to protect you.
I think if you return, you should steel yourself for some revelations abut his life. It does sound to me that he is not as poor and as desperate as he wants you to think, which has to be good news on the maintenance front at least.
You have made a life for yourself and your dd across the other side of the world to home - I admire you a lot. You sound like your return to Australia will be well thought out. You will have a job to go to and lovely inlaws waiting. So there are lots of positives. Hope the negatives are small in comparisom - keep posting on mumsnet if you want. There are some regular posters based in Australia, one of whom has lots of legal knowedge - bloss, where are you?