If you’ve lodged a Business Activity Statement and spotted a mistake, the ATO gives you two ways to fix it: correct the error on your next BAS, or revise the original statement. Which route is open to you depends on whether the error means you underpaid or overpaid, how much money is involved, how long ago you lodged, and — for larger businesses — your GST turnover. Knowing how to correct BAS errors after lodgment matters because the wrong choice can expose you to penalties and interest that the right choice would have avoided.
Start by working out what kind of error you have. A credit error means you overpaid GST or under-claimed credits, so the ATO owes you. A debit error means you underpaid or over-claimed, so you owe the ATO. Credit errors are treated more leniently. Debit errors face tighter time limits and dollar caps because they involve revenue the government should have already collected.
One boundary before going further: this framework covers mistakes — things that were wrong when you lodged. It doesn’t cover adjustments, which are events that happened after lodgment (a price reduction, a returned item) and follow their own rules.
Fixing a Credit Error on Your Next BAS
Credit errors are the simpler case. You can correct one on a later BAS as long as that later BAS is lodged within the four-year period of review for the original reporting period. The four years start the day after you lodged the BAS containing the error. There’s no dollar cap — the full amount can be corrected regardless of size.
Two limits to be aware of. If the credit error involves GST credits you never claimed, the separate four-year time limit for claiming those credits still applies, and once it’s gone the credit is gone. And if the ATO has already begun a compliance activity — an audit, review, or verification check — that covers the reporting period, you can’t use the later-BAS method at all. You’ll need to raise the error directly with the officer handling the review.
Fixing a Debit Error on Your Next BAS
Debit errors have to clear two hurdles: a time limit and a dollar limit, both driven by your current GST turnover.
- Turnover under $20 million: the debit error must be corrected on a BAS lodged within 18 months of the due date of the original BAS, and the net amount of all debit errors being corrected for that period must be less than $12,500.
- Turnover of $20 million or more: the correction window shortens to 12 months from the due date of the original BAS. Value limits scale with turnover, reaching less than $560,000 for entities turning over $1 billion or more.
The dollar limit applies to the net sum of all debit errors you’re correcting for a single earlier period, not to each error on its own. If the combined total exceeds your cap, the later-BAS route is closed and you have to revise the original.
Two more disqualifiers apply regardless of turnover. You can’t use the later-BAS method if the error resulted from recklessness or intentional disregard of a GST law, and you can’t use it while the ATO is running a compliance activity that covers the relevant period.
Where the Correction Goes on the BAS
When you correct a GST error on a later BAS, the correction amount goes into either label 1A or label 1B. If the net effect is that you underpaid GST, include the correction at 1A (GST on sales). If you overpaid, include it at 1B (GST on purchases).
For every correction, keep a written note recording the reporting period where the error was made, the BAS on which you corrected it, the dollar amount, and a short explanation of what went wrong. That note sits alongside your regular GST records and is your evidence trail if the ATO ever asks.
Revising the Original BAS
When the later-BAS route isn’t open to you — because of timing, dollar limits, an active compliance activity, or the nature of the error — you revise the original statement instead. A revision replaces the original figures for that reporting period.
Online
In Online Services for Business, go to Lodgments, then Activity statements. Find the relevant period in your history, select “Revise” next to the processed statement, and enter the corrected amounts over the top of the originals the system displays. Keep the confirmation receipt.
Through a Registered Agent
A registered tax or BAS agent can lodge the revision for you through Online Services for Agents or the Practitioner Lodgment Service, provided their registered number is linked to your records.
On Paper or by Letter
If you don’t use online services, contact the ATO for a paper revised activity statement, fill in only the items being revised (plus any items affected by the change), and mail it back in the pre-addressed envelope. You can also write a letter asking for the amendment. Include your ABN, the tax period, details of the error, the correction amount, supporting documents, and a signed declaration that the information is true and correct.
PAYG, Fuel Tax Credits, and WET
A BAS covers more than GST, and the other components have their own quirks.
Fuel tax credits follow the same structure as GST: 18-month or 12-month debit error windows by turnover, the same dollar caps, no cap on credit errors within the four-year period of review, and the same disqualifiers for compliance activity or reckless conduct.
Wine equalisation tax uses the same credit/debit framework but with higher debit caps — for example, up to $20,000 for entities with turnover under $20 million, compared to $12,500 for GST. WET adds one restriction: you can’t correct the same error more than once. If you’ve already corrected a WET error in another reporting period, you can’t correct it again on a later BAS. Supporting records for WET corrections must be kept for five years.
PAYG instalments work on a different clock. You can fix an instalment amount on or before the day the instalment is due, or before you lodge your income tax return for that year, whichever comes first. Once the return is lodged, the instalment figures are locked in. PAYG withholding errors follow a separate process — check with the ATO or your agent for the right amendment path.
Penalties, Interest, and Voluntary Disclosure
Not every mistake attracts a penalty. If you took reasonable care preparing your BAS and still got it wrong, the ATO’s position is that no penalty applies. The penalty system targets behaviour, not outcomes.
Where conduct falls short of reasonable care, the penalties escalate:
- Failure to take reasonable care: 25% of the shortfall.
- Recklessness: 50% of the shortfall, applied when a reasonable person in your position would have seen the risk and you ignored it.
- Intentional disregard: 75% of the shortfall, reserved for cases where you knew of a clear obligation and deliberately chose not to comply.
These base penalties can be lifted by 20% if you have a history of similar penalties, obstructed the ATO, or knew about the shortfall and didn’t disclose it within a reasonable time. Where the shortfall is less than $1,000 and you disclose voluntarily before any compliance activity starts, the penalty drops to nil.
Interest is separate. The general interest charge applies to any tax unpaid after its due date, including shortfalls uncovered by BAS corrections. For the first quarter of 2026 the GIC rate is 10.65% per annum, rising to 10.96% for the second quarter, compounding daily. It runs from the original due date of the underpaid tax, not from when you found the error, so a correction lodged two years late carries two years of accumulated interest. The ATO can remit all or part of the GIC where the late payment stemmed from events outside your control and is inconsistent with your usual compliance history; requests tend to be viewed more favourably where the GIC amount is $2,500 or less and your prior record is clean.
Voluntary Disclosure
Telling the ATO before they find it themselves cuts your penalty significantly. The size of the reduction depends on timing:
- Before you’re notified of an audit: penalties are reduced by 80%, and drop to nil if the shortfall is under $1,000.
- After notification but during a review, if the ATO invites a disclosure and you respond within the deadline: typically still an 80% reduction.
- After an audit has begun without an ATO invitation: 20%, and only if the disclosure can reasonably be estimated to have saved the ATO significant time or resources.
For activity statement errors — GST, fuel tax credits, WET, and PAYG withholding — the ATO treats the correction or revision of the BAS itself as the voluntary disclosure. You don’t necessarily need to lodge a separate form. If you’re already under audit, though, raise the error directly with the reviewing officer and use the voluntary disclosure form designed for taxpayers under review.
Safe Harbour When a Tax Agent Made the Error
If a registered tax or BAS agent prepared the statement and made the mistake, you may be shielded from penalties entirely. The safe harbour rule applies where three things are true: you gave the agent all relevant taxation information needed to prepare the statement correctly, the agent made the statement, and the agent didn’t act recklessly or with intentional disregard of the law.
The onus is on you to show you provided everything, and the test is objective. If you genuinely thought you’d handed your agent the full picture but actually left out a material document or gave them an incorrect summary, safe harbour doesn’t apply. Keep a clear record of what you provided and when — it’s the best protection you have.
The Four-Year Outer Limit
All of the above sits inside a four-year period of review set by the Taxation Administration Act 1953. It starts the day after you lodge the BAS. Once it expires, the figures on that BAS are final — you can’t claim further credits for the period, and (outside of fraud) the ATO can’t increase your liability.
The debit-error windows for correcting on a later BAS — 18 months or 12 months — sit well inside that four-year envelope. Missing them doesn’t extinguish the debt; it just closes off the simplified route. You still have to revise the original statement, and you still have to pay the correct amount.
The only exception to the four-year outer limit is fraud or evasion. Where the Commissioner forms that opinion, the ATO can amend assessments indefinitely. In practice this is reserved for deliberate schemes, not for honest mistakes corrected in good faith.