A compliance letter from the ATO rarely means a human has looked at the file. It usually means an automated system has cross-referenced the BAS against another dataset it already holds and found a gap. The ATO's data-matching runs the same specific comparisons on every return, so the errors that generate letters are predictable, and each one corresponds to a check a firm can run before lodgement instead of after the letter arrives. Knowing exactly which comparisons the ATO makes is what turns this from a source of anxiety into a short pre-lodgement checklist.
The cross-checks the ATO's systems actually run
The most consistent triggers are mismatches between datasets the ATO holds simultaneously. Income reported on the BAS that does not reconcile to the income tax return for the same period. PAYG withholding on the BAS that does not match the wages reported through STP. Bank deposits that do not line up with declared sales. Any of these can generate an automated letter with no human review, because the system is built to compare these figures as a matter of course. The second mechanism is industry benchmarking. The ATO publishes financial ratios by industry, covering things like cost of goods to turnover, labour costs against revenue, and gross profit margins, and a business sitting outside its benchmark range without an obvious reason flags for review. This is especially common in cash-heavy sectors such as construction, hospitality, and the trades, where a tradesperson whose reported income looks low against their bank deposits is exactly the pattern the system is designed to catch.
The accounting-method mismatch is a less obvious trigger but a real and easily prevented one. A business with aggregated turnover under $10 million can choose to report GST on a cash basis, meaning GST is reported when payment is received or made rather than when invoices are issued. The trap is a mismatch between the software and the ATO registration: if the accounting software is set to accrual while the client is registered with the ATO as a cash reporter, every BAS reports on the wrong basis, and the ATO can detect it because the timing of the figures does not match what it expects from a cash reporter. This one is entirely avoidable by confirming the software's GST basis matches the client's registered method, and the only proper fix once it has happened is to correct the registration and revise the affected BAS periods.
The errors that show up most often
Wrong GST category coding is the most common individual error, and the usual culprits are claiming GST credits on wages and superannuation, which carry no GST, mishandling overseas software subscriptions, and applying GST to GST-free supplies or the reverse. Each of these flows straight into the 1A or 1B fields and produces a figure the ATO can test against benchmarks and third-party data. Missed or incorrect PAYG withholding is the second most frequent, usually when a client pays a contractor without checking whether an ABN was quoted, or when a bonus or back-pay is run outside the normal payroll cycle and captured in neither the STP submission nor the BAS. Because the ATO cross-references employer STP data against employee tax return data automatically, a withholding figure that does not match what employees declare flags at both ends at once. The third is large period-on-period variance without a business reason: a business reporting $200,000 in sales one quarter and $80,000 the next will draw attention unless there is a visible explanation such as seasonality, a completed project, or a genuine change in activity. A firm that notes and can explain significant variances in the file before lodging is in a far stronger position if the ATO asks the question.
The fifteen minutes that prevents the letter
Finding a BAS error after lodgement does not mean an audit is inevitable, but it does mean the window for fixing it cheaply is closing. The reason speed matters is the penalty framework: where an error results in a shortfall, the base penalty for a simple failure to take reasonable care is 25% of the shortfall, but a voluntary disclosure made before the ATO makes contact reduces that base penalty by 80%, which brings the effective penalty on a reasonable-care error down to around 5% of the shortfall. Left until the ATO finds it, the same shortfall carries far more, and deliberate errors sit at the top of the scale. The habit that prevents most of this is a short pre-lodgement reconciliation: confirm that BAS income reconciles to the accounting records, that PAYG figures match the STP submissions, and that the accounting method in the software matches what the ATO has on file. Those three checks take about fifteen minutes, they map directly onto the exact comparisons the ATO's systems run, and they catch the errors that generate compliance letters before the letter is ever sent.
