I doubt that you couldn't take the matter to the AAT (but stranger things have happened). Did the AAT tell you they couldn't look at it? If so, you can appeal it to a court.
Have a read of this if you haven't already to see if this information helps: 2.6.14 Reason 8 - a Parent's Income, Property, Financial Resources, or Earning Capacity | Child Support Guide
Have a read of this if you haven't already to see if this information helps: 2.6.14 Reason 8 - a Parent's Income, Property, Financial Resources, or Earning Capacity | Child Support Guide