How GST treats discounts

Whether a discount reduces GST depends on when it is agreed.

Known at the time of supply

If the discount is recorded on the invoice (or agreed before the sale), it is deducted before GST is applied, so you pay tax on the lower amount.

Given later

Post-sale discounts only reduce GST if they were agreed in advance and can be linked to specific invoices, usually through a credit note. Random later discounts do not reduce the original tax.