Bookkeeping errors rarely look serious when they first occur. A transaction is coded incorrectly, a receipt is misplaced or GST is claimed on a purchase that did not include GST. Over several months, however, these small mistakes can lead to incorrect business activity statements, unreliable financial reports, cash-flow problems and unexpected tax liabilities.
The Australian Taxation Office continues to focus on small businesses that fail to keep accurate records, report all income or manage their GST and PAYG obligations. In February 2026, the ATO reported that mistakes contributed to Australia’s $27.2 billion small-business income tax gap and urged businesses to improve their record keeping and cash-flow management.
Here are some of the most common BAS, GST and bookkeeping mistakes business owners should avoid.
1. Waiting Until BAS Time to Update the Books
Leaving bookkeeping until the end of the quarter creates unnecessary pressure. Bank feeds may contain unexplained transactions, invoices may be missing and the business owner may struggle to remember whether a payment was business-related or private.
Bookkeeping should be updated and reviewed regularly, preferably weekly or monthly, depending on transaction volume. Bank accounts, credit cards, loans and payment platforms should be reconciled before the BAS is prepared.
Regular bookkeeping also gives business owners a more reliable picture of sales, expenses, debtors, creditors and available cash. A BAS should be prepared from reconciled accounting records, not estimated from bank balances.
2. Claiming GST Where No GST Was Charged
Not every business expense includes GST. Common examples of amounts that may not include claimable GST are:
- Bank fees and interest
- Stamp duty
- Employee wages
- Superannuation contributions
- Income tax payments
- Government fines and penalties
- Purchases from suppliers not registered for GST
- GST-free goods and services
A business cannot claim a GST credit simply because an expense was paid from its business account. The transaction must be a creditable acquisition, and the supplier must generally have charged GST.
The ATO also requires a valid tax invoice to claim a GST credit for purchases of $82.50 or more, including GST, subject to limited exceptions.
3. Claiming the Private Portion of Business Expenses
Many expenses have both business and private components. Common examples include motor vehicles, mobile phones, internet services, travel and home-office costs.
Only the business-use portion is generally claimable. If a vehicle is used 70% for business and 30% privately, the GST credit and income tax deduction may need to be apportioned accordingly.
Business owners should retain supporting calculations, logbooks and other records showing how the business percentage was determined. Automatically claiming 100% of a mixed-use expense can overstate both the GST refund and the income tax deduction.
4. Using the Wrong GST Code
Modern accounting platforms make bookkeeping easier, but they do not remove the need to classify transactions correctly.
A common mistake is applying the standard GST code to every receipt or payment. Transactions may instead be taxable, GST-free, input-taxed, outside the GST system or subject to special rules.
For example, basic food may be GST-free, while bank interest is generally input-taxed. Wages are not reported as purchases for GST purposes, and transfers between business bank accounts should not be treated as income or expenses.
Sales and business asset disposals also need the correct treatment. The sale or trade-in of equipment, machinery or a motor vehicle may need to be reported at G1 Total sales and 1A GST on sales where the transaction is taxable.
5. Reporting an Invoice Twice
Duplicate transactions can arise when an invoice is manually entered and then the payment imported through the bank feed is recorded as a separate expense.
The same issue can occur with sales when an invoice is recorded in the accounting system but the customer’s payment is later coded as additional income rather than matched against the invoice.
Duplicates overstate income, expenses or GST. They also make accounts receivable and accounts payable reports unreliable. Bank-feed transactions should be matched with existing invoices or bills before new transactions are created. The ATO specifically recommends ensuring invoices are counted only once.
6. Using the Wrong GST Accounting Method
A business may account for GST on a cash or non-cash basis.
Under the cash method, GST is generally reported when payment is received or made. Under the non-cash method, GST is generally reported when an invoice is issued or received, even if no payment has occurred.
Eligible small businesses with aggregated turnover below $10 million may generally choose the cash method. Businesses need to ensure their accounting software, bookkeeping processes and BAS preparation follow the method registered with the ATO.
From 1 July 2026, the ATO began moving some businesses to the correct reporting arrangements. A business with GST turnover of $10 million or more generally needs full BAS reporting and non-cash GST accounting, while businesses with GST turnover of $20 million or more generally report GST monthly.
7. Missing the GST Registration Threshold
A business must generally register for GST when its GST turnover reaches, or is expected to reach, $75,000. Registration is generally required within 21 days after the business becomes required to register.
The threshold is based on GST turnover, not taxable profit. A business can therefore make a small profit or even a loss and still be required to register.
Taxi, limousine and ride-sourcing operators generally need to register regardless of turnover. Failing to register on time can result in GST being payable from an earlier date, even if the business did not add GST to customer invoices.
8. Mixing Business and Personal Spending
Using one account for business and private spending makes reliable bookkeeping more difficult. It can also create problems when calculating GST credits, income tax deductions, director loan accounts and business cash flow.
Private expenses should not be treated as normal business deductions. For companies, private payments made for shareholders or their associates may also create Division 7A consequences if they are not handled correctly.
Separate business bank accounts and credit cards provide a clearer audit trail. The accounting records should also identify drawings, director loans and private expenses correctly rather than hiding them among operating costs.
9. Treating GST and PAYG Withholding as Available Cash
GST collected from customers and PAYG withholding deducted from employees’ wages will ultimately need to be paid to the ATO. Spending these amounts on day-to-day operations can leave a business unable to meet its BAS obligations.
The ATO recommends putting money aside for GST and PAYG withholding and using separate accounts where appropriate. It also encourages businesses experiencing difficulty to seek assistance early rather than ignoring overdue liabilities.
This has become even more important because general interest charge and shortfall interest charge incurred from 1 July 2025 are no longer income tax deductible.
10. Failing to Prepare for Payday Super
From 1 July 2026, employers must pay superannuation guarantee contributions for each payday instead of relying on the previous quarterly payment framework.
Contributions generally need to reach the employee’s super fund within seven business days after payday, subject to limited exceptions. Employers must also report qualifying earnings and super liabilities through Single Touch Payroll.
The Small Business Superannuation Clearing House has also closed for payments from 1 July 2026. Businesses need a suitable alternative payment provider and payroll processes capable of meeting the shorter timeframe.
Correcting BAS and GST Mistakes
Discovering an error does not necessarily mean the business must wait for an ATO review. Some GST errors can be corrected in a later BAS if the relevant conditions and time limits are satisfied. Other errors may require revision of the original activity statement.
The correct approach depends on whether the mistake is a debit or credit error, its value, when it occurred and whether it resulted from deliberate conduct. Businesses should avoid reversing or recoding transactions without retaining a clear record of the correction.
Better Bookkeeping Produces Better Decisions
Accurate bookkeeping is not only about satisfying the ATO. Reliable records help business owners understand profitability, monitor overdue invoices, manage expenses and plan for upcoming tax payments.
At a minimum, businesses should regularly:
- Reconcile bank accounts, credit cards and loans
- Review outstanding customer invoices and supplier bills
- Check GST classifications
- Separate private expenditure
- Review payroll and superannuation liabilities
- Set aside funds for BAS and tax payments
- Investigate unusual or duplicated transactions
A structured monthly review can prevent small errors from becoming expensive problems.
Frequently Asked Questions
Q1. What are the most common BAS mistakes?
Common mistakes include claiming GST on GST-free expenses, using incorrect GST codes, duplicating transactions, including private expenses and failing to reconcile the accounts before lodging.
Q2. Can I correct a mistake on a previous BAS?
Some GST mistakes can be corrected on a later BAS if the ATO’s eligibility conditions and time limits are met. Otherwise, the original BAS may need to be revised.
Q3. How long should GST records be kept?
Businesses should generally retain tax invoices and other GST records for five years. Longer retention periods may apply to some transactions or disputes.
Q4. Do I need a bookkeeper to prepare my BAS?
A business owner may maintain their own records, but anyone providing BAS services for a fee must meet the applicable registration requirements. Using a qualified professional can help identify coding, reconciliation and reporting errors before lodgement.
BAS, GST and Bookkeeping Support
Accurate books make BAS lodgements easier and reduce the risk of unexpected tax liabilities. Kintax Accountants assists small businesses with bookkeeping reviews, BAS and GST preparation, payroll, account reconciliations and ongoing taxation and compliance requirements.
Contact Kintax Accountants
Phone: 0399393692
Email: info@kintax.com.au
Office: Level 1, 287A Spring Street, Reservoir VIC 3073
Enquiries: Request a consultation
Based in Reservoir, Kintax Accountants supports small businesses throughout Melbourne’s northern suburbs and greater Melbourne.
Important information: This article provides general tax, accounting and bookkeeping information only. It does not constitute tax, financial or legal advice specific to your circumstances. Obtain professional advice before acting on this information.
