In the first instance you should approach the Child Support Agency with a view to having them stop the Child Support Assessment. Sometimes the Child Support Agency will not stop collecting Child Support from you. In that situation it is likely that you will need to commence a court proceeding seeking a stay of the Child Support Assessment and an Order for DNA testing to occur.
Whether or not you will be entitled to a refund of the Child Support which has been paid will depend on the circumstances of your particular matter.