Fixing a BAS mistake: revise it, or fix it on the next one.

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First: mistake or adjustment?

The ATO treats these as different things, and it decides which fix you use. A mistake was wrong on the day you lodged: a transposed figure, a missed invoice, GST claimed on a GST-free purchase, a sale reported twice. An adjustment is a later event that changed a number that was correct when you reported it: a customer cancelled and got a refund, a price was renegotiated, a debt you'd paid GST on went bad. Adjustments aren't corrections at all. You simply report them on the BAS for the period the event happened, and you're done. Everything below is about genuine mistakes.

The two ways to fix a mistake.

• Correct it on a later BAS. Fold the fix into the next statement you lodge: one lodgement, no reopening old periods, and no penalty on the corrected amount. This is the default whenever the error qualifies.
• Revise the original BAS. Re-lodge the period with the right figures through ATO online services or your agent. Required when the error doesn't qualify for the shortcut, and often worth doing anyway when the original period shows a large overpayment you want refunded now rather than netted off later.

Credit errors: the generous rules.

A credit error made your GST bill too high: you double-counted a sale, forgot GST credits you were entitled to, or reported GST on a GST-free supply. These have no value limit: you can correct a credit error of any size on a later BAS, as long as that BAS is lodged within the four-year period of review of the original one. Remember the parallel deadline, too, GST credits themselves generally expire if unclaimed four years after the due date of the BAS they first belonged on.

Debit errors: capped and timed.

A debit error made your GST bill too low. The kind the ATO is stricter about. For a business with GST turnover under $20 million, you can fix a debit error on a later BAS only if:

Larger businesses get higher value limits on a sliding scale, see the ATO's correcting GST errors tables. If you have several errors in both directions, you can net the debit errors off against the credit errors and test only the net amount against the cap. A debit error over the cap, or older than the window, means revising the original BAS. The shortcut is off the table.

When you can't use the fix-it-later shortcut.

• The error came from recklessness or intentional disregard, deliberate under-reporting isn't an "error" the shortcut forgives. Those must be handled as a voluntary disclosure on a revision.
• The ATO is already reviewing the period. Once you've been notified of an examination covering that BAS, corrections for it can't quietly ride on a later statement: disclose directly instead, which still earns significant penalty reductions.
• It's outside the time or value limits above, revise the original.

Penalties and interest: the honest picture.

Fixing an eligible error on a later BAS attracts no penalty and no interest on the corrected amount. The ATO treats it as reported in the later period. A voluntary revision that increases what you owe can technically attract shortfall penalties and general interest from the original due date, but penalties are generally reduced to nil where you took reasonable care and came forward before any audit. The expensive path is the third one: leaving it for the ATO's data-matching to find. If cash is the worry rather than the paperwork, the same rule as late BAS applies: fix the lodgement first, then ask for a payment plan.

How to actually lodge the fix.

• Correcting on a later BAS: just adjust the labels on your next statement: add under-reported GST to 1A, add missed credits to 1B (refresher on the labels). Keep a working paper showing what the correction was; it's your evidence the netting stayed under the cap.
• Revising: in Online services for business (or myGov for sole traders), open the lodged statement and choose to revise it; agents lodge revisions through their own portal. The revised figures replace the originals for that period.
• Not sure which? Ring the ATO, or have a registered agent look at it: a five-minute classification (mistake vs adjustment, credit vs debit) decides the whole process.

Quick answers.

Related guides: What is a BAS? · BAS due dates · BAS calculator · How GST credits work · Tax invoice requirements · GST record-keeping · GST on discounts and rebates · Closing your business · GST on entertainment expenses