Hi,
Do you have Joseph's marriage cerificate showing his father was Joseph?
I have found the passenger list at 
Voyage Arrival Date: 11 Dec 1838 
Vessel Name: James Paterson 
AUSTEN  Joseph    22    Rolvenden    Carpenter    Protestant    Reads & Writes    
             Sophia    26    Tenterden    Housemaid          "                     "
             Lewis       3         "                                    "
Also couldn't the Joseph death in 1890 be this son of Joseph and Sophia born...
Joseph AUSTEN
Birth Date: 1853 
Father's Name: Joseph  
Mother's Name: Sophia 
Birth Place: New South Wales 
Registration Place: Ni, New South Wales 
Daisy