Made a mistake on a lodged BAS? How to fix it without making it worse

First, is it a mistake or an adjustment?

A mistake is something that was wrong on the day you lodged: a sale left out, a purchase entered twice, GST claimed on something that carried none, a number typed into the wrong label. An adjustment is something that was right when you lodged but has since changed: a refund, a cancelled sale, a debt written off after twelve months, or an asset that shifts from mostly business to mostly private use.

Adjustments are not corrections. They go on the BAS for the quarter the change happened, at 1A or 1B as appropriate, and you never touch the old statement. See bad debts and GST for the most common one. Everything below is about actual mistakes.

Two ways to fix a mistake

You can either correct the error on a later BAS, or revise the original. The ATO lets you use the first, simpler route if the error is within the time and value limits below, it was not the result of recklessness or deliberately ignoring the law, and the ATO has not already told you it is reviewing that period.

Correcting on a later BAS means no separate form, no interest on the shortfall and no penalty. Revising means the ATO recalculates that quarter, and if you owe more, interest runs from the original due date. Given the choice, correct on a later BAS.

Credit errors: you paid too much

A credit error is one where fixing it reduces your net GST: you forgot to claim GST on a purchase, reported a sale twice, or reported a GST-free sale as taxable. You can correct these on any later BAS within four years of the due date of the original statement, and there is no dollar limit. Increase 1B (or reduce 1A) on the later statement by the amount of the error, and note in your records which quarter it related to and why.

Debit errors: you paid too little

A debit error is one where fixing it increases your net GST: a sale left out, GST claimed on a bank fee or the stamp duty on an insurance premium, or a personal purchase claimed as business. These have two limits, and both depend on your GST turnover. For a business with turnover under $20 million, which is every sole trader reading this:

• Value limit: the net debit error you correct on one BAS can be up to $10,000. Net means the debit errors less any credit errors from the same earlier period that you are correcting at the same time.
• Time limit: the later BAS you correct on must be lodged within 18 months of the due date of the original.

Check the ATO for the current limits before relying on them. If your error is over the value limit or the time has passed, you must revise the original statement.

How to revise a BAS

Sign in to ATO online services through myGov (or Online services for business), go to Tax, then Lodgments, then Activity statements, open the history and choose Revise next to the period. Change only the labels that were wrong, lodge, and the ATO issues a revised assessment. A BAS or tax agent can do it for you. See how to lodge BAS online for finding your way around the portal.

If the revision means you owe more, the general interest charge applies from the original due date until you pay. Penalties for a false or misleading statement can apply in principle, but a voluntary revision of an honest error by a small business is usually met with no penalty or a reduced one; telling the ATO before they find it is what earns that treatment. If the revision means a refund, the ATO may ask for supporting documents first.

What it costs

Corrected on a later BAS within the limits: nothing beyond the GST itself. Revised: the GST plus the general interest charge, which compounds daily at a rate set each quarter (check the ATO for the current figure) and which, since 1 July 2025, is not tax deductible. On a $3,000 shortfall found a year late that is a few hundred dollars. Not fixing it at all is the expensive option: if the ATO finds it in a review, the interest is the same and the penalties are not.

The mistakes we see most, and how to stop repeating them

GST claimed on things that never carried it: bank fees, loan interest, insurance stamp duty, ASIC fees, payments to unregistered subcontractors. GST on sales forgotten because income was entered without it. Deposits counted once from the invoice and again from the bank statement. A whole quarter lodged as nil because the software showed nothing.

Working from the bank statement fixes most of these, because every dollar in and out is there exactly once. FlowFi builds G1, 1A and 1B from the statement you upload, marks the lines it wants you to confirm, and shows the running position all quarter, so the BAS you lodge is one you have already looked at. The labels guide explains what each figure should contain.