BAS Lodgement Guide 2026: 9 Costly Mistakes Australian Small Businesses Must Avoid

If you run a GST-registered business in Australia, the Business Activity Statement (BAS) is one of those unavoidable quarterly rituals. Get it right and it’s a routine piece of admin. Get it wrong and you’re looking at penalties, interest charges and unwanted attention from the ATO.

With the ATO’s penalty unit rising to $364 from 1 July 2026 — and interest charges no longer tax-deductible — mistakes cost more in 2026 than ever before. This guide walks you through the nine most common BAS lodgement mistakes we see small businesses make, and how to avoid every one of them.

BAS Due Dates 2026–27: Know Your Deadlines

Most small businesses with GST turnover under $20 million lodge quarterly. Missing these dates is the single most expensive mistake on this list.

*28 February 2027 falls on a Sunday, so the Q2 deadline shifts to Monday 1 March 2027.

Monthly BAS lodgers (turnover above $20 million) must lodge by the 21st of the following month. Put these dates in your calendar now — or better yet, let a registered BAS agent manage them for you and pick up the extra four weeks of lodgement time.

Mistake 1: Missing the Lodgement Deadline

It sounds obvious, but late lodgement is the most common BAS error in Australia. Life gets busy, the quarter slips by, and suddenly you’re weeks overdue.

The cost is real: the ATO can issue a Failure to Lodge penalty of one penalty unit ($364 from 1 July 2026) for every 28 days your BAS is overdue, up to five units — that’s $1,820 for a single late statement. And if you owe money, the General Interest Charge accrues daily at around 11.43% per annum.

Mistake 2: Getting GST Classifications Wrong

Not every sale and purchase attracts GST — and coding them incorrectly is a fast track to an ATO adjustment. The three categories trip up even experienced bookkeepers:

• Taxable supplies — GST applies (most goods and services).
• GST-free supplies — no GST charged, but you can still claim GST credits (e.g. fresh food, exports, most medical services).
• Input-taxed supplies — no GST charged and no GST credits allowed (e.g. residential rent, financial services).

Claiming GST credits on input-taxed purchases, or forgetting to charge GST on taxable sales, both lead to shortfalls the ATO will eventually catch.

Mistake 3: Claiming GST Credits on Personal Expenses

The business bank card doesn’t make a purchase business-related. Family holidays, personal groceries and the new TV for the living room are not GST-creditable expenses — even if they were paid from the business account.

Mixed-use expenses (like a car or mobile phone used for both business and personal purposes) must be apportioned. You can only claim the business-use percentage.

Mistake 4: Lodging from Unreconciled Books

A BAS prepared from three months of unreconciled transactions is essentially a guess. Missed transactions, duplicates and bank feeds that stopped syncing weeks ago all flow straight into your BAS figures — and straight into ATO data-matching.

Mistake 5: Skipping “Nil” BAS Lodgements

Had a quiet quarter with no activity? You still have to lodge. The ATO expects a BAS for every period you’re registered — a “nil” lodgement takes minutes online, but skipping it triggers the same Failure to Lodge penalties as any other missed BAS.

Mistake 6: Cash vs Accrual Accounting Confusion

Your BAS must be prepared on the same accounting basis you elected for GST — cash or accrual — and many businesses don’t know which one they’re on.

• Cash basis: you account for GST when money actually changes hands.
• Accrual (non-cash) basis: you account for GST when you issue or receive an invoice, even if it hasn’t been paid.

Small businesses with turnover under $10 million can generally choose cash accounting, which is simpler and better for cash flow. But if your software is set to accrual while you think in cash terms, your BAS figures won’t match your expectations — or your bank balance.

Mistake 7: Ignoring PAYG Instalments

Your BAS isn’t just about GST. If you pay PAYG instalments, they’re reported on the same statement — and many business owners either forget them or vary them incorrectly.

You can vary your instalments if your income has changed significantly, but varying without a reasonable basis (or varying down to avoid paying) can attract its own penalty. And ignoring instalments altogether just defers the pain to tax time.

Mistake 8: Spending the GST You’ve Collected

This isn’t a lodgement error — it’s the mistake that makes lodgement painful. GST you collect from customers is not your money; you’re holding it on behalf of the ATO. Businesses that treat it as working capital face a nasty surprise every quarter when the BAS bill arrives and the cash is gone.

Mistake 9: Going It Alone Without a BAS Agent

DIY BAS lodgement is perfectly legal — but it’s not always wise. If your GST position is complex, you have employees, or you simply keep missing deadlines, professional help pays for itself. A registered BAS agent:

• Gives you around four extra weeks on most quarterly deadlines
• Reviews your GST coding and picks up errors before the ATO does
• Keeps your books reconciled year-round so BAS time is painless
• Handles ATO correspondence and payment plans if things go wrong

Your Pre-Lodgement Checklist

Before you hit “lodge”, run through this list:

• All bank accounts reconciled for the quarter
• GST coding reviewed (taxable, GST-free, input-taxed)
• Personal expenses excluded; mixed-use items apportioned
• PAYG withholding figures match your STP payroll reports
• PAYG instalment reviewed against actual income
• Figures cross-checked against the previous quarter for anomalies
• Lodged and paid by the due date

What Happens If You Get It Wrong?

Beyond the Failure to Lodge penalties ($364 per 28 days overdue, up to $1,820 per statement for small businesses), incorrect reporting can attract shortfall penalties of 25% to 75% of the tax understated, depending on the behaviour involved. General Interest Charges compound daily on unpaid amounts — and since 1 July 2025, they’re no longer tax-deductible.

Repeated issues also increase your chances of ATO review or audit — a process that typically consumes 20 to 40 hours of a business owner’s time.

Frequently Asked Questions

When is BAS due in 2026–27?

Quarterly BAS is due 28 October 2026 (Q1), 1 March 2027 (Q2), 28 April 2027 (Q3) and 28 July 2027 (Q4) for self-lodgers. Registered agents lodging electronically get extended dates for Q1, Q3 and Q4.

What is the penalty for late BAS lodgement?

From 1 July 2026, the ATO can charge one penalty unit ($364) per 28-day period overdue, up to five units ($1,820) per statement for small businesses. Interest also accrues on unpaid amounts.

Do I need to lodge BAS if I had no business activity?

Yes. If you’re registered for GST, you must lodge every BAS period — including nil activity periods. Lodge a nil statement online in minutes.

Should I use a BAS agent or lodge myself?

If your GST affairs are simple and you never miss deadlines, DIY is fine. If you have employees, complex GST coding, or a history of late lodgement, a registered BAS agent saves you time, stress and penalties — plus around four extra weeks per quarter.

Can I correct a BAS mistake after lodging?

Yes. You can revise an activity statement through the ATO’s online services or via your BAS agent. Correcting voluntarily before the ATO finds the error generally results in lower penalties.

This article is general information only and doesn’t constitute tax advice. For advice tailored to your circumstances, speak to a registered tax or BAS agent.