BAS Mistakes That Trigger ATO Audits (and How to Avoid Them)

The ATO audits around 2% of small businesses each year. That sounds low until you realise their data matching systems flag thousands of BAS returns automatically. Certain errors show up like a flare.

Here are the mistakes that draw attention, and what to do differently.

Mistake 1: Claiming GST on GST-Free Items

Not everything you buy has GST on it. If you claim a GST credit on a GST-free purchase, you're overclaiming, and the ATO's systems catch it.

Common GST-free categories people trip on:

• Fresh food: Unprocessed fruit, vegetables, meat, bread, milk. If you buy lunch supplies for a work event, the sandwich platter has GST but the fruit platter might not.
• Medical services: GP visits, physiotherapy, most health services. If your business pays for employee health checks, there's no GST to claim.
• Education and training: Most accredited courses are GST-free. A short course from a private provider may include GST, but a TAFE certificate won't.
• Residential rent: If you work from a home office and pay rent, the rent itself is GST-free. You can't claim a GST credit on it.

Check the tax invoice. If it doesn't show a GST amount, don't claim one.

Mistake 2: Mixing Personal and Business Expenses

The ATO expects a clear line between personal and business spending. Running everything through one bank account makes this harder and raises red flags.

The fix is simple: get a separate business bank account and a separate business credit card. Only put business expenses through them. If you buy something personal by accident, don't claim it. Mark it as a personal expense in your accounting software and move on.

The ATO specifically watches for patterns like fuel purchases on weekends (personal driving coded as business) and grocery store transactions (household shopping claimed as business supplies).

Mistake 3: Vehicle Claims Without a Logbook

If you claim more than $5,000 in vehicle expenses per year, the ATO expects you to have a logbook. The logbook must cover at least 12 consecutive weeks and show every trip: date, start odometer, end odometer, purpose, and kilometres.

A valid logbook lasts for 5 years, as long as your driving pattern hasn't changed significantly. "Significantly" means more than about 10% difference in business use percentage.

Mistake 4: Forgetting to Include Cash Sales

Every dollar your business earns goes on your BAS, whether the customer paid by card, bank transfer, or cash. The ATO cross-references your reported income against bank deposits, payment terminal data, and industry averages.

If you're a tradie who occasionally gets paid in cash, record it the same day. Write it in a cash sales book or enter it directly into your accounting software. The ATO treats unreported cash income as tax fraud, not an honest mistake.

Mistake 5: Wrong GST Codes in Your Accounting Software

Xero, MYOB, and QuickBooks assign GST codes to transactions automatically based on the category you choose. The defaults are usually right, but not always.

Watch for these common miscodes:

• Bank fees: Should be GST-free (code: FRE), but often default to GST (code: GST)
• Insurance: Some insurance premiums include a stamp duty component that has no GST. Your software might code the entire amount as GST-inclusive.
• Government fees: ASIC fees, council permits, and licence renewals are often GST-free but get auto-coded as GST
• Overseas purchases: Software subscriptions from US companies (like Adobe or Canva) may or may not include GST depending on the supplier's Australian registration

Before you lodge, run a GST detail report and scan for anything that looks off. Five minutes of checking saves a potential audit.

Mistake 6: Not Reconciling Before Lodging

Lodging your BAS without reconciling your bank feeds first is like submitting a job quote without checking the material prices. You'll get the numbers wrong.

Reconcile every transaction for the period before you lodge. Match bank transactions to invoices. Clear out anything sitting in the "For Review" pile. If a transaction doesn't have a receipt, find it or exclude the GST claim.

Mistake 7: Full GST Claims on Luxury Vehicles

You can't claim full GST credits on motor vehicles that cost more than the luxury car tax threshold. For fuel-efficient vehicles, that threshold was $76,950 in the 2024-25 financial year (it adjusts annually).

If you buy a $100,000 ute for the business, you can only claim the GST credit up to the threshold amount, not on the full purchase price. The maximum GST credit is roughly $6,995 (one-eleventh of $76,950), regardless of what you actually paid.

This catches people who buy high-end vehicles and claim the full $9,090 GST credit on a $100,000 purchase. The ATO flags it automatically.

What Actually Triggers an ATO Audit

The ATO doesn't pick businesses at random. Their data matching systems look for specific patterns:

Large GST Refunds Relative to Income

If your business reports $80,000 in sales but claims $15,000 in GST refunds, that ratio looks unusual. Refunds that are disproportionately high compared to your reported income get flagged for review.

Sudden Changes in Reported Figures

Your GST collected drops 40% from one quarter to the next but your industry hasn't changed. Or your GST credits suddenly double without a corresponding increase in sales. The ATO's systems track your historical patterns and flag sharp deviations.

Industry Benchmarks

The ATO publishes performance benchmarks for dozens of industries. They know, for example, that a plumber with $300,000 in revenue typically has cost of sales around 25% to 35%. If your numbers sit well outside the benchmark range for your industry, expect a closer look.

You can check your own industry benchmarks on the ATO's small business benchmarks page.

Data Matching from Third Parties

The ATO receives transaction data from banks, payment processors (Square, Stripe, Tyro), government agencies, insurance companies, and property registries. If your reported sales don't match what your bank or payment terminal shows, that discrepancy creates an automatic flag.

What to Do If You Get an Audit Letter

First: don't panic. Most BAS audits are "desk audits" where the ATO asks for specific documents by mail or email. They're not raiding your office.

1. Read the letter carefully. It will tell you exactly what they want: specific invoices, bank statements, or GST calculations for a particular period.
2. Respond by the deadline. You usually get 28 days. If you need more time, call the number on the letter and ask for an extension. They almost always grant it.
3. Get your BAS agent or accountant involved. They deal with ATO audits regularly and know what format the ATO prefers. This is not the time to DIY it.
4. Provide only what they've asked for. Don't volunteer extra information or send your entire accounts file. Answer the specific questions.
5. If you've made an error, own it. The ATO treats voluntary disclosures far more favourably than discovered errors. Penalties are reduced by up to 80% if you come forward before they find the mistake.

A Quick Checklist Before You Lodge

• Bank feeds reconciled for the full period
• GST codes reviewed on all transactions
• Cash sales recorded and included
• Personal expenses excluded
• Vehicle logbook current (if claiming vehicle expenses)
• No GST claimed on GST-free purchases
• Motor vehicle GST capped at luxury car threshold
• Figures compared to last quarter for any large swings

Run through this list every quarter. It takes ten minutes and keeps you off the ATO's radar. For more on the lodgement process itself, see Lodging Your First BAS: A Plain English Walkthrough. For all the 2026 deadlines, we've got those covered too.