Every quarter — or every month, depending on your turnover — the BAS is due. And every quarter, finance teams across Australia go through the same process: pull the figures from Xero or MYOB, review the GST totals, hope the numbers look right and lodge.
The problem is that by the time you are looking at the BAS, the errors have already happened. They happened weeks or months ago, one invoice at a time, when someone coded a supplier bill with the wrong GST treatment. The BAS just aggregates those errors into a single number that either overstates or understates your GST position.
For businesses processing hundreds of invoices a month — construction companies, wholesale distributors, freight operators, manufacturers — the volume of invoices makes it almost certain that some will be coded incorrectly. The question is not whether you have GST coding errors. It is how many you have and whether they are material.
Where the errors start
GST coding errors in accounts payable fall into a handful of common patterns. Most of them are not the result of carelessness. They are the result of invoices being more complicated than the coding process allows for.
Claiming GST on GST-free supplies
A supplier sends an invoice for $1,100. The person processing the invoice assumes it includes GST and codes it as GST-inclusive, claiming a $100 input tax credit. But the supply was GST-free — the supplier is not registered for GST, or the goods fall into a GST-free category. The $100 credit should not have been claimed.
This is the single most common GST coding error in accounts payable. It happens because the default behaviour in most accounting systems is to apply GST, and the person processing the invoice does not check the supplier’s GST registration status or the nature of the supply.
Missing GST credits on valid invoices
The opposite error. A supplier sends a valid tax invoice with GST clearly stated. The person processing the invoice codes it as GST-free — perhaps because the amount is small, or they are unsure, or they are being conservative. The business misses a legitimate input tax credit.
This error does not trigger ATO penalties, but it costs money. A business that systematically under-claims GST credits by 2 to 3 percent of its input tax is overpaying GST every quarter.
Mixed invoices coded with a single GST treatment
A freight invoice has six line items. Four are taxable. Two are GST-free — perhaps an international leg of the shipment or a government charge that is exempt. The person processing the invoice does not want to split it across multiple tax codes, so they apply one treatment to the whole invoice.
If they code the whole invoice as GST-inclusive, they overclaim on the two GST-free lines. If they code it as GST-free, they miss credits on the four taxable lines. Either way, the BAS figure is wrong.
This is particularly common in construction, freight and wholesale businesses where multi-line invoices with mixed GST treatments are routine rather than exceptional.
Imported goods and deferred GST
Businesses that import goods can defer GST on imports and report it on their BAS rather than paying it at the border. The GST is then claimed as an input tax credit on the same BAS, resulting in a net zero effect.
But the coding has to be right. The deferred GST needs to be reported as both a liability (G11 on the BAS) and a credit (G15). If the customs broker invoice is coded as a regular domestic purchase with standard GST, the deferral is not reported correctly and the BAS figures do not reconcile.
For businesses importing regularly — e-commerce brands, wholesale distributors, manufacturers sourcing materials from overseas — deferred GST coding errors can accumulate to material amounts over a few quarters.
Supplier GST registration changes
A supplier was GST-registered when you set them up in your system two years ago. They have since deregistered — perhaps their turnover dropped below the $75,000 threshold, or they restructured. Your system still applies the GST-inclusive tax code to their invoices because nobody updated the supplier record.
Every invoice from that supplier is now coded incorrectly. You are claiming input tax credits on invoices from a supplier that is no longer charging GST. This is an overclaim and it is exactly the kind of error the ATO looks for in audits.
Why these errors compound
A single GST coding error on a $500 invoice creates a $50 discrepancy. That is not material. But the same error pattern applied across dozens of invoices from the same supplier, or the same type of invoice, across a full financial year — that becomes material.
The compounding effect is particularly dangerous because the errors are consistent. If your team always codes freight invoices with a single GST treatment instead of splitting the lines, the error repeats on every freight invoice. If your system does not check supplier GST registration status, every invoice from a deregistered supplier carries the same error.
By the time the BAS is prepared, these systematic errors are buried in the aggregated GST figures. The total GST claimed looks plausible. The total GST collected looks right. But the underlying data has errors that only surface during an ATO audit or a thorough internal review.
The BAS review problem
Most businesses rely on a BAS review process to catch these errors before lodgement. In practice, the review looks like this:
- Pull the GST transaction report from Xero or MYOB
- Scan the report for obvious anomalies — unusually large GST claims, unexpected GST-free amounts
- Check a sample of high-value invoices
- Compare the total GST position to last quarter
- Lodge
This process catches large, obvious errors. It does not catch systematic small errors. If 15 freight invoices were each over-claimed by $80, the total $1,200 discrepancy does not show up as an anomaly in a transaction report with thousands of lines.
The other problem is timing. The BAS review happens after the fact — weeks or months after the invoices were processed. The person reviewing the BAS is not the person who coded the invoices. They do not have context on why a particular invoice was coded a certain way. They are looking at numbers in a report, not invoices on a desk.
Fixing the problem at the source
The only reliable way to prevent GST coding errors from reaching the BAS is to catch them at the point of invoice processing — not during BAS review.
This means validating the GST treatment of every invoice before it is posted to the accounting system:
Check supplier GST registration. Before applying a GST-inclusive tax code to an invoice, verify that the supplier is currently registered for GST on the Australian Business Register. This is a live lookup — not a check against stale data in your supplier master file.
Validate at the line-item level. Do not apply a single GST treatment to a multi-line invoice. Each line item should be assessed individually. If line 3 is GST-free and lines 1, 2 and 4 are taxable, the coding should reflect that.
Flag mismatches. If a supplier is not GST-registered but the invoice shows a GST amount, flag it. If a supplier is GST-registered but the invoice does not show a GST amount, flag it. These mismatches are either errors on the supplier’s invoice or errors in your supplier data — either way, they need to be resolved before posting.
Handle deferred GST correctly. For imported goods, ensure the system applies the correct deferred GST treatment — reporting the liability and the credit on the same BAS. This is a specific coding pattern that should be automated for businesses that import regularly.
Re-check supplier registration periodically. Supplier GST registration status changes. A quarterly check against the ABR for all active suppliers catches deregistrations before they result in months of incorrect coding.
The cost of getting it wrong
The ATO penalties for incorrect GST reporting are structured as a percentage of the shortfall:
- 25 percent of the shortfall for failure to take reasonable care
- 50 percent for recklessness
- 75 percent for intentional disregard
For most AP coding errors, the 25 percent penalty applies — the business did not intentionally report incorrectly, but it also did not have adequate processes to prevent the error.
On top of the penalty, the ATO charges a general interest charge on any underpaid GST from the original due date. For errors that span multiple quarters, the interest charge alone can be significant.
But penalties are not the only cost. An ATO review or audit triggered by GST discrepancies consumes significant internal time. The finance team has to pull records, explain coding decisions and respond to ATO queries — often for invoices processed months ago by people who may no longer be with the business.
And there is the cash flow impact of overclaiming. If you have been claiming GST credits you were not entitled to, the ATO will require repayment of the overclaimed amount plus interest and penalties. For a business that has been systematically overclaiming by $2,000 to $3,000 per quarter for two years, the total repayment can be $20,000 to $30,000 before penalties.
What a clean AP-to-BAS process looks like
For businesses processing high volumes of invoices with mixed GST treatments, the AP-to-BAS process should work like this:
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Invoice arrives. The system captures the invoice and extracts line-item data including amounts, descriptions and any GST amounts shown on the invoice.
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Supplier validation. The system checks the supplier’s ABN against the ABR to confirm current GST registration status. If the registration status has changed since the last invoice from this supplier, it flags the invoice for review.
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Line-item GST coding. Each line item is coded with the appropriate GST treatment based on the supplier’s registration status, the nature of the supply and the business’s GST accounting method. Mixed invoices are split correctly across tax codes.
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Mismatch detection. If the GST treatment applied by the system does not match what the supplier has shown on the invoice — for example, the supplier shows GST but is not registered — the invoice is flagged as an exception.
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Approval and posting. Once validated, the invoice moves through the approval workflow and is posted to Xero or MYOB with correct GST coding on every line item.
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BAS preparation. When BAS time comes, the GST figures in the accounting system are already clean. The BAS review becomes a confirmation step rather than an error-hunting exercise.
The difference between this process and the typical manual process is where the validation happens. In a manual process, validation is attempted at step 6 — during BAS review. In a structured process, validation happens at steps 2, 3 and 4 — before the invoice is posted.
Practical steps for the next BAS period
If you want to reduce GST coding errors before your next BAS lodgement, start with these steps:
Run an ABR check on your top 50 suppliers. Go to the Australian Business Register and confirm that every supplier you are claiming GST credits from is currently registered for GST. If any are not registered, review every invoice from that supplier in the current BAS period.
Review all invoices over $5,000 for correct GST treatment. High-value invoices have the largest impact on your GST position. Check that the tax code applied matches the supplier’s GST status and the nature of the supply.
Identify your mixed-invoice suppliers. Which suppliers regularly send multi-line invoices with different GST treatments on different lines? Freight companies, customs brokers and large materials suppliers are common examples. Check that these invoices are being split correctly rather than coded with a single tax treatment.
Check your import GST coding. If you import goods and use the deferred GST scheme, pull a sample of customs broker invoices and verify that the deferred GST is being reported correctly on your BAS — both as a liability and as a credit.
These checks take time when done manually. But they are significantly less time-consuming than responding to an ATO review after lodging an incorrect BAS. And for businesses processing hundreds of invoices a month, they make a strong case for automating GST validation at the invoice processing stage rather than relying on manual review at BAS time.
Further reading: Best AP Automation Software Australia 2026 · What a Modern AP System Needs to Do · The Real Cost of Manual AP