Made a Mistake on Your BAS? How to Fix a Lodged BAS in Australia

Most BAS mistakes are found the same way: a bookkeeper reconciles the next quarter, or an accountant prepares the year-end GST reconciliation, and the numbers do not agree with what was lodged. The good news is that the ATO expects errors and has a set process for fixing them. The key is choosing the right method and keeping a record of what you changed.

This guide explains the options in plain terms. The exact time and value limits depend on your GST turnover and the type of error, so check the ATO's "Correcting GST errors" guidance or ask your registered BAS or tax agent before you lodge a correction.

Common BAS mistakes we find

• GST claimed on items with no GST: bank fees, interest, insurance stamp duty, many government charges, and overseas purchases without Australian GST
• Sales coded as GST-free or exports when they were taxable, or the reverse
• Duplicate supplier bills, or the same bill entered from both a receipt app and the bank feed
• Capital purchases coded as ordinary expenses, or left out of G10 for businesses reporting the full set of GST labels
• Private spending on a business card claimed in full
• Wages at W1 and PAYG withheld at W2 that do not match the payroll records or STP
• A whole bank account, merchant facility or PayPal account never reconciled for the quarter

Option 1: correct a GST error on a later BAS

For many GST errors you do not need to touch the original statement. If the error is within the ATO's time limit and value limit, you can include the correction in the BAS you are preparing now. An extra GST credit you missed (a credit error) can usually be claimed in a later BAS within the four-year period. GST you under-reported (a debit error) can be corrected on a later BAS only if it falls within the limits that apply to your turnover.

In practice this means adding the adjustment to the relevant GST labels in the current period and keeping a working paper showing the original period, the amount, and why it changed. In Xero or MYOB, post the correction as a dated adjustment with a clear note, not by editing transactions in a period that has already been lodged and locked.

Option 2: revise the original BAS

If an error is outside the limits for correcting later, or you simply prefer a clean record, you revise the activity statement for the period where the mistake happened. You or your agent do this through ATO online services or the agent portal. Interest can apply to any extra GST owing from the original due date, so revising sooner reduces the cost.

Revising is also the safer choice when the same mistake runs across several quarters. Spreading corrections over later statements makes the audit trail harder to follow for you, your accountant and the ATO.

PAYG withholding errors work differently

The GST correction rules do not automatically apply to W1, W2 or other PAYG withholding labels. Wages reported through Single Touch Payroll also need to match, so a fix often starts in the payroll software with an STP update event, followed by a revision of the activity statement where needed. Ask your agent which approach fits before you change payroll figures on a BAS.

Voluntary disclosure and penalties

If an error meant you paid less tax than you should have, telling the ATO before it contacts you generally leads to much lower penalties than an error found in a review. Correcting on a later BAS or revising the original both count as fixing it; leaving a known error in place does not. Interest charges may still apply to unpaid amounts.

A five-step process for fixing a BAS error

• Find the cause: reconcile the period first, so you know whether it is one transaction, a coding rule, or a missing account
• Measure it: total the GST effect for each period affected, separating extra GST owed from extra credits
• Choose the method: correct on a later BAS if within the limits, otherwise revise the original, and confirm with your agent if unsure
• Record it: keep a working paper with the period, amount, labels affected and reason
• Fix the source: change the bank rule, GST default or payroll setting that caused it, so the error does not repeat next quarter

Why errors surface in the first quarter

The July to September BAS, due 28 October for most quarterly lodgers, is often where last year's problems show up. Year-end adjustments, the annual GST reconciliation and STP finalisation all happen just before it, and each one compares a full year of figures. If the year-end work found a difference, Q1 is usually the first chance to deal with it.

Preventing the next one

Most repeat BAS errors come from settings, not people: a bank rule with the wrong GST code, a supplier set to "GST on expenses" by default, or a pay item mapped to the wrong account. A quarterly GST reconciliation that compares the BAS with the GST account in the ledger catches them early.

We prepare BAS drafts with that reconciliation inside your Xero, MYOB or QuickBooks file, and list anything that looks like a prior-period error for you or your agent to decide on. We are not a registered BAS agent and do not lodge or revise statements; that stays with you or your agent.