How do I go about getting my childrens birth father to sign over his rights?

I live in Western Australia and my children are 7 and 5. Thier biological father hasnt seen them since they were 3 and 1. He owes over $20,000 in child support and has never called or sent cards on birthdays or chritmases. His family are the same and have nothing to do with the children. He was extrememely abusive which is why I finally left. I just found out that he has moved to another state and his last girlfriend took out a 2yr violence restraining order against him. I have been in a relationship now for 4 years with a great man who treats me and the kids like gold. We recently had a baby and he would like to adopt my kids but I would need my ex to consent bysigning his rights and responsibilities over. Has anyone been in this situation before and know how to go about it? Thanks.