Correct a GST mistake: later BAS or original revision?

Keep the lodged business activity statement (BAS) unchanged while you investigate. Save its transaction report, identify the affected entries and calculate the GST difference separately from the gross sale or purchase amount.

Choose the right starting question

The ATO distinguishes GST errors from later adjustments and unclaimed credits. Those categories have different rules. A refund agreed after a correctly reported sale is different from entering the sale twice before lodgement.

1. Was the original GST amount wrong?

Yes: calculate the GST error and continue to the correction checks. No: consider a later commercial event or a previously unclaimed credit.

2. Did a later event change a correct transaction?

A return, cancellation or price change may need an adjustment and supporting note. Follow the adjustment rules instead of treating every change as an original error.

3. If it is an error, can the later-BAS rules be used?

Check error direction, time, net debit value, compliance restrictions and whether it has already been corrected. Record the evidence for each condition.

4. Record the selected route

If eligible, identify the later BAS and the correction amount. Otherwise have the agent check original-period revision and any interest, penalties or other procedural requirements.

This is a review sequence, not an automatic eligibility result. Sources: ATO error guidance and the Correcting GST Errors Determination 2023 linked below. Original ASBG diagram with a complete text equivalent in each step; fictional examples.

Check debit and credit errors separately

A debit error means too little net GST was reported or paid; a credit error means too much. The current Correcting GST Errors Determination 2023 is the source for later-period correction conditions. For current GST turnover below $20 million, the published debit-error rules use a later BAS lodged within 18 months of the original due date and a net debit amount below $12,500. Other turnover bands have different limits. Check the instrument again at the time of correction, including how multiple errors are combined.

Credit errors have no equivalent dollar ceiling under these rules, but the later BAS must be lodged within the original assessment's period of review. That period is different from the separate four-year limit for an unclaimed purchase credit. The ATO period-of-review guidance explains when the assessment clock starts.

Also check whether the error concerns a matter under ATO compliance activity, has already been corrected, or arose from recklessness or intentional disregard. These conditions matter even when the dollar amount is small. Don't divide a large error across several later BAS returns to get around a limit.

Work through a small duplicate claim

Fictional example: a $3,300 fully taxable purchase was claimed twice, so 1B included $600 instead of $300. The debit error is $300, not $3,300. Removing the duplicate reduces the credit and increases net GST by $300.

The review log records the original BAS period and due date, lodgement receipt, both ledger IDs, valid supplier invoice, calculation, current turnover band and proposed later lodgement date. It also records the agent's confirmation of the other conditions. The $300 amount passes the value check in this example; the other conditions still need answers.

If the duplicate was $33,000 instead, the GST error would be $3,000, assuming the same wholly taxable purchase. Gross transaction size and GST error size should never share an unlabelled column.

Preserve the link between books and lodged returns

After the correction is approved, record the chosen route and retain both the old report and corrected report. Reconcile any software prior-period adjustment to that decision. The software finding an edited transaction doesn't itself authorise reporting it in the next BAS.

If you need an original-period revision, follow the current ATO process for that statement and save the new receipt. The online BAS guide explains lodgement access. Use the BAS-agent versus tax-agent guide if the issue extends beyond the agent's scope.

Continue with the next question

• Adjustment notes when a sale changes: the GST paper trail
• Found an old receipt? Check the unclaimed GST credit

Key takeaways

• Separate an original error, a later adjustment and a missed credit.
• Test every later-BAS condition, not just the dollar threshold.
• Keep one evidence log showing where the amount was finally corrected.

Where to get help

Read the ATO correction guide, including its worked conditions and have a registered agent confirm the route. Related preparation guides are in Bookkeeping and BAS.