How the GST instalment amount is calculated
The GST instalment amount printed at box G21 on your activity statement is generally based on the actual GST net amounts you reported in your activity statements or most recent annual GST return.
If you are newly registered for GST, the net amount for your first quarter is excluded from the calculation of your GST instalment. This is because the net amount for your first quarter might be affected by start-up costs.
Your GST instalment amounts may change during the year if you vary your instalment amount or lodge an annual GST return.