Wanted to reiterate what Lulu said: tell CSA you wish to claim child support through them. They then keep track of what they collect from him on your behalf, and if he tries to shirk his responsibilities, they will garnish his wages. It doesn't matter what sort of relationship (or lack thereof) you two have - he has to pay. If he does not pay, CSA will do all the legal stuff on your behalf, and the FAO/Centrelink gets informed that he's not paying. He can't escape really.