If you are a * GST instalment payer, your * net amount for an * instalment tax period is the difference between:
(a) the amount that, but for this section, would be your * net amount under section 17 - 5, 123 - 15 or 126 - 5 for the instalment tax period; and
(b) the sum of all of the * GST instalments payable by you for the * GST instalment quarters of the instalment tax period.