(1) The Registrar must amend a notional assessment of the annual rate of child support that would be payable for a child (the first child ) for a particular day in a child support period if:
(a) another child in the child support case that relates to the first child is not covered by the relevant child support agreement or the order that was made in relation to the first child; and
(b) the administrative assessment of the child support payable for the other child for any day (the changed assessment day ) in any child support period changes.
(2) The Registrar must amend the notional assessment as if:
(a) despite subsection 146BA(2), section 67A (offsetting) applied on the changed assessment day; and
(b) the annual rate of child support that would be payable for the first child for the particular day were instead payable for the changed assessment day.