Found a mistake on a BAS you've already lodged? Don't panic. Most GST mistakes can be fixed on your next BAS by adding the correction at label 1A (you paid too little GST) or 1B (you paid too much). If the error is too old, too big, or isn't a GST error at all (like a PAYG withholding reporting mistake), you revise the original BAS instead.
Below, in plain English: which kind of mistake you've made, the ATO's time and value limits, when to revise, and how to stop it happening again. If you'd rather hand the whole thing over, our BAS and IAS lodgement service is built for exactly this.
First, breathe: BAS mistakes are common and fixable
A wrong GST code, an invoice entered twice, a taxable sale coded GST-free: these happen every quarter, especially when the books are rushed. The ATO has a set process for fixing them, and it generally says correcting a GST error on a later BAS is easier than revising an earlier one, which can attract penalties or the general interest charge (GIC). So it pays to know which path you're allowed to take.
What counts as a "GST error"?
The ATO defines a GST error (ATO: correcting GST errors) as a mistake relating to an amount of GST, a GST credit or a GST adjustment. There are two types:
Credit errors (you paid too much GST)
A credit error means the GST net amount you reported made you pay too much GST for that period. Common causes the ATO lists include:
- reporting a GST sale twice
- overstating the GST on sales (for example, reporting a bigger GST on sales figure than the correct amount), where the refund isn't restricted
- leaving out or understating a decreasing GST adjustment
- overstating an increasing GST adjustment
Debit errors (you paid too little GST)
A debit error means you reported or paid too little GST. Common causes include:
- not including GST on a taxable sale
- understating the GST on sales
- overstating GST credits, such as claiming the GST on the same purchase twice
- leaving out or understating an increasing GST adjustment
- overstating a decreasing GST adjustment
What isn't a GST error
A few things look like mistakes but are handled differently:
- A GST credit you simply forgot to claim. In the ATO's own worked example, a missed credit for a purchase isn't treated as a GST error, so it's claimed on a later BAS without running the error tests. You still need to claim it within the 4-year credit time limit.
- Adjustments. If something changed after the sale (a refund, a bad debt, a price change), that's a GST adjustment, not an error. Adjustments go on the BAS for the period the change happened.
- PAYG withholding and PAYG instalment mistakes. These aren't GST errors and follow their own rules. More on these below.
Option 1: Correct it on your next BAS
If your mistake is a GST error and it meets the ATO's conditions, you fix it on the BAS you're preparing now. No revision, no paper form.
How it works:
- Work out the net effect of all the GST errors you're correcting. The ATO's "net sum of debit errors" is your debit errors minus any credit errors included on the same BAS.
- If the net result means you paid too little GST, add the correction amount at 1A (GST on sales).
- If the net result means you paid too much GST, add it at 1B (GST on purchases).
- Keep a record of the period the error was made, the BAS you corrected it on, and the supporting documents that explain the correction.
The conditions for credit errors
The ATO says credit errors have no value limit. You can correct one on a later BAS as long as that BAS is lodged within the period of review for the period where the error was made. The period of review starts on the day you lodge the BAS and ends 4 years and one day later.
The conditions for debit errors
Debit errors have three tests. Each debit error must:
- be within the debit error time limit
- keep the net sum of debit errors within the debit error value limit
- not be the result of recklessness or intentional disregard of GST law
Debit error time limit. The clock runs from the due date of the BAS where the error was made, not from when you found it. According to the ATO:
- if your current GST turnover is less than $20 million, you must correct it on a BAS lodged within 18 months of the due date of the BAS where the error was made
- if your current GST turnover is $20 million or more, the limit is 12 months
Watch out for this trap: the ATO's examples show that lodging your current BAS late can push you outside the time limit. If the window closes, you need to revise the original period instead.
Debit error value limit. For most small businesses the first line is the one that matters. The ATO's types of GST errors page sets the limits:
- GST turnover less than $20 million: net debit errors less than $12,500
- $20 million to less than $100 million: less than $25,000
- $100 million to less than $500 million: less than $50,000
- $500 million to less than $1 billion: less than $100,000
- $1 billion and over: less than $560,000
If your net debit errors go over the limit, you can correct up to the limit on your latest BAS and revise the earlier period for the rest. You can't carry the excess forward to another later BAS.
A simple example
Say you're a sole trader with GST turnover well under $20 million, lodging quarterly. While preparing your September quarter BAS, your bookkeeper notices a supplier bill from last March quarter was entered twice, and you claimed $400 in GST credits you weren't entitled to. That's a debit error.
It's within 18 months of the March BAS due date, it's well under the $12,500 value limit, and it was a genuine slip. So you add $400 at 1A on the September BAS, note "March quarter duplicate bill corrected" in your file, keep the bill and the reconciliation, and you're done. (This is an illustration, not advice for your specific situation.)
Option 2: Revise the original BAS
You'll need to revise the BAS for the period where the mistake happened if:
- the debit error is outside the time limit (18 or 12 months, depending on your turnover)
- the net debit errors are over the value limit (for the excess amount)
- the error came from recklessness or intentional disregard
- the mistake isn't a GST error, such as reporting PAYG withholding wrongly
- the period is the subject of an ATO compliance activity (the ATO's rules check this before you can self-correct)
If you're not sure which category you're in, that's a good moment to talk to your registered BAS agent or tax agent before you lodge anything.
How to revise a BAS
The ATO's revising an earlier BAS page says you can revise online through:
- Online services for business: go to Lodgments, then Activity statements, then History, and select Revise next to the statement you need to fix. Enter the revised values and lodge.
- ATO online services for individuals and sole traders (through myGov)
- Online services for agents, if a registered tax or BAS agent lodges for you
The ATO notes that if the Revise button doesn't appear, you can't revise that statement online and you'll need to contact them for a paper version. They also ask you not to send a copy generated from commercial software.
One more thing: if changing one label changes others, update those too. For example, correcting wages at W1 and tax withheld at W2 also flows through to summary labels like W5, 4, 8A and 9.
PAYG withholding mistakes are different
PAYG withholding has its own correction rules, and they depend on what went wrong.
- You withheld the right amount but reported it wrongly (or didn't report it): for amounts not reported through Single Touch Payroll (STP), you lodge a revised activity statement. If you overstated PAYG withholding, that's also a revision.
- You didn't withhold, or withheld the wrong amount from a payment: the ATO says to write to them. Don't put an amount you failed to withhold on an original or revised activity statement.
- The mistake is in your STP reporting: you may also need to fix your STP data, and if you've already made a finalisation declaration, submit an update event so your employees' income statements are right. The ATO also allows some STP-related PAYG withholding corrections to be carried forward to the current period, within limits.
The ATO says that when a revision increases the tax you owe, or you tell them by letter that you didn't withhold, it's generally treated as a voluntary disclosure, which usually means concessional treatment of penalties and interest. In plain English: owning up early is far better than waiting to be found.
If payroll mistakes are piling up, our STP and Payday Super qualifying earnings guide explains the reporting side in more detail.
Not sure whether your mistake is a quick next-quarter fix or a revision? Get a free quote or book a discovery call and we'll take a look. No judgement. We've seen it all.
How to stop BAS mistakes happening again
Most BAS mistakes start in the books, weeks before lodgement. A few habits stop most of them:
- Reconcile bank feeds weekly or fortnightly, not once a quarter. Duplicates are easier to spot in small batches.
- Check GST codes on new suppliers and income types. One wrong default tax rate repeats every month.
- Review the GST report before you lodge for big GST-free amounts, unusual credits or wrong-period dates.
- Set a lock date in Xero after each BAS is lodged, so filed periods can't be edited by accident.
- Keep a simple corrections log: date found, period affected, amount, label used, and where the documents are.
- Lodge on time. Late lodgement can shrink your window for correcting debit errors on a later BAS. If you use a registered BAS agent, you may also be able to access the ATO's BAS agent lodgment program concessions. Our BAS due dates 2026–27 guide has the details.
For the Xero walkthrough, see how to prepare and lodge your BAS in Xero. If the mistakes go back more than a quarter or two, catch-up bookkeeping is usually the fastest way to get clean again.
Where a BAS agent helps (and where you need a tax agent)
As a registered BAS agent and Xero Certified Advisor, Girl Friday Australia can work out what kind of error you've got, check the ATO limits, correct it on your next BAS or prepare a revision, and fix the coding behind it.
If the mistake touches your income tax, such as how a correction affects your tax return, deductions, or business structure, speak to your tax agent or accountant. That's outside BAS-agent scope, and it's worth getting right.
Frequently asked questions
Can I fix a mistake on my BAS in the next quarter?
Usually, yes, if it's a GST error. Credit errors (you paid too much) can be corrected on a later BAS lodged within 4 years and one day of lodging the original. Debit errors (you paid too little) must also be within the ATO's time and value limits. Use label 1A or 1B for the net correction.
How long do I have to correct a GST debit error?
If your current GST turnover is under $20 million, you must correct it on a BAS lodged within 18 months of the due date of the BAS where the error was made. At $20 million or more, the limit is 12 months. After that, you revise the original period.
Will I get a penalty for revising my BAS?
It depends on your situation. The ATO notes revising an earlier BAS can incur penalties or GIC. For PAYG withholding, it says a revision that increases what you owe is generally treated as a voluntary disclosure. Talk to your BAS or tax agent about your case.
How do I revise a BAS online?
In Online services for business, go to Lodgments, then Activity statements, then History, and select Revise next to the statement. Sole traders can also use ATO online services through myGov, and registered agents can revise through Online services for agents.
Can Girl Friday fix a BAS mistake for me?
Yes. We're a registered BAS agent and can review the error, correct it on your next BAS or prepare a revision, and clean up the books behind it. Call 0404 877 436, email info@girlfridayaustralia.com, or book a discovery call.
Fix it once, then keep it fixed
A BAS mistake isn't a disaster. It's a signal that something in the books needs a tweak. Correct it the right way, keep a clear record, and tighten the habits that caused it.
If you'd like someone to check your last few BAS statements and tell you plainly what needs fixing, we're here. Get a free quote or start the conversation. Australia-wide, no lock-in contracts, and we reply within 1 business day.