When you report on your activity statement depends on the accounting basis you use. You can account on a cash basis or a non-cash basis.
You need to account for GST when using the margin scheme as follows.
G1 Total sales
Report the amount of the 'margin' on your sale at G1.
If the margin is nil (or a negative amount), don't report any amount at G1 (total sales).
1A GST on sales
Report the amount of GST on your margin at 1A (GST on sales).
Only report the GST on the sale on the BAS at label 1A, not the withholding amount paid by the purchaser.
Purchases
If you buy property and the GST included in the price was worked out using the margin scheme, you aren't entitled to a GST credit for the purchase.
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