The BAS can be set either by cash or accrual basis.
If Tax Reporting is on Cash Basis, only realized transactions will be displayed in the Business Activity Statement/ GST Report. While accrual reporting will pick up transactions based on their date/period regardless whether the transaction has been realized or has been paid or not.
You can check on this via Setup>Accounting>Setup Taxes
Once you have checked the tax reporting selected, you also need to consider if an invoice is linked to an advance payment, The invoice date/period is not being picked up in BAS for invoices linked to advance payments/credits application in cash basis, instead the system only calculates taxes for paid invoices and include this in BAS based on the date/period of the advance payment.
For illustration purpose, invoice IN00150897 was invoiced and paid in the same month of June, if the tax is based on cash basis , the system recognizes sale using the date of the payment or credit transaction link to the invoice. If accrual basis, the recognition of sales is automatically on the date of the invoice and not when the date when its paid. 
Another example is Invoice IN00150862s, invoice date is Dec 2019 but it was only paid in Sept 2020, if cash basis reporting is in place, this invoice is included in the computation of sales on BAS at the time it was paid on Sept 3, 2020, although if you look at your P/L for Sept 2020, this is not included as sales because it is part of the sales in Dec 2019.
Things that you need to remember when it comes to BAS
- The figures showing on your BAS vs your profit and loss will be dependent on the tax reporting selected.
- If accrual basis, the report takes the date of the invoice, paid or not.
- If cash basis : if the invoice is linked to advance payments/credits application, the date on the linked payment will be used in the reporting.
