Does the biological father have parental responsibility? If he does, he may be able to object to the removal of his son from the country, or you may have to apply to take him out of the country. If he does not have parental responsibility, he would have to apply for that first in order to be able to object. What I am thinking is, that there may be 2 issues for you - one is whether Australia want permission from bio father for son to migrate- the answer to that is likely no, I would think. The other issue may be whether the biological father has recourse in the family courts to try to prevent you, or whether in the family law system whether you need permission to take the child out of the country. So it hinges on the parental responsibility I would think - you need to get advice from CAB / a family lawyer to work out what the situation is. I'm rusty on all of that but worked in family law for a bit. I know enough to know you would be wise to know what you are dealing with. Knowledge is power. If you know the score, then if ex partner tries to put you off or interfere, you will be equipped to deal with him emotionally and practically.