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If their BAS is prepared on a cash basis, and they receive payment for invoices that were issued before deregistration, is GST still payable on the amounts received after deregistration?
Correct procedure would be to include GST payable on all tax invoices issued before the GST cancellation date, in your final BAS, whether they have been paid or not.
Then if they are paid after the BAS reporting period the GST component isn't payable (as it has already been remitted to the ATO)