September is the final month of Quarter 1. Your books are still open, transactions are still rolling in, and the BAS deadline feels comfortably far away. Then October arrives, and suddenly 28 October is a week out.
Most small business owners at this point are scrambling to reconcile three months of activity in a hurry. That’s exactly when mistakes happen uncoded transactions slip through, GST on a big purchase gets claimed twice, payroll figures don’t match what was reported through Single Touch Payroll. The ATO notices these things.
The uncomfortable truth: the ATO’s data-matching capability is more sophisticated than most business owners realise. Discrepancies between your BAS figures and third-party data (bank feeds, STP submissions, supplier reports) are flagged automatically. An amended return is an inconvenience. A formal review is something else entirely.
A clean BAS isn’t about luck. It’s about the checks you run before you lodge. This guide covers the key Q1 2026-27 dates, the seven pre-lodgement checks that catch the errors most likely to attract ATO attention, and why engaging a registered BAS agent before 28 October is worth more than the extra month it buys you.
Q1 2026-27 Key Dates at a Glance
Quarter 1 covers 1 July to 30 September 2026. The ATO generated your Q1 activity statement on 6 September 2026, meaning it has been available in your ATO Online Services or accounting software since mid-September.
Here are the dates that matter:
| Ledger type | Lodgement and payment due date |
|---|---|
| Self-lodger (paper or online) | 28 October 2026 |
| Registered BAS agent (electronic via OSfA or PLS) | 25 November 2026 |
Source: ATO BAS agent lodgment program 2026-27 and Activity statement generate dates.
Who qualifies for the 25 November extended deadline?
The extended deadline applies only when a registered BAS agent lodges electronically on your behalf through the ATO’s Online Services for Agents (OSfA) or the Practice Management Software (PLS) channel. It is not available if you lodge through your own myGov or ATO Online Services account, even if a BAS agent helped you prepare the return.
Important: the extended deadline applies to both lodgement and payment. If you owe GST or PAYG withholding for Q1, the payment is also not due until 25 November when a registered agent lodges on your behalf.
If you are currently self-lodging and your books are not in order, this is the most practical reason to engage a registered BAS agent before 28 October. You gain nearly four additional weeks to get the numbers right, without incurring a failure-to-lodge penalty.
The 7 Pre-Lodgement Checks Every Small Business Should Run
These are the checks that separate a BAS lodged in confidence from one lodged in hope. Run through each one before you submit, and you eliminate the most common sources of ATO queries and amended returns.
| # | Check | What to look for | Common error |
|---|---|---|---|
| 1 | GST accounting method match | Confirm whether you are on cash or accrual basis. Your method should be consistent with prior quarters and match what is registered with the ATO. | Switching between methods mid-year without formal notification creates GST figures that cannot be reconciled by the ATO. |
| 2 | Uncoded and unreconciled transactions | Run a report for all transactions without a GST code. Any item coded “no GST” should be genuinely GST-free or input-taxed, not just unreviewed. | Bank feed imports often leave transactions uncoded. These are excluded from your GST calculation entirely, understating either your GST collected or your input tax credits. |
| 3 | STP reconciliation: W1 and W2 | Your W1 (total salary and wages) and W2 (PAYG withholding) figures must match the year-to-date totals reported through Single Touch Payroll. | A mismatch between BAS W1/W2 and STP data is one of the most common triggers for an ATO review. Payroll adjustments, termination payments, and manual journal entries are frequent culprits. |
| 4 | GST on large or unusual purchases | Review any significant purchases made in Q1. Confirm the correct GST treatment: fully taxable, GST-free, or input-taxed. | Claiming full GST credits on items with mixed private and business use (e.g. a vehicle used partly for personal travel) overstates your input tax credits. |
| 5 | Nil BAS rule awareness | If your business had no taxable activity in Q1, you are still required to lodge a nil BAS by the due date. | Many business owners assume a nil return is not required. A failure-to-lodge penalty applies regardless of whether any tax is owed. |
| 6 | Fuel tax credits | If your business uses fuel in machinery, heavy vehicles, or off-road equipment, check whether you have claimed the correct fuel tax credit rate. The ATO adjusts rates twice yearly. | Using an outdated rate (the rate changed in February 2026) results in either an underclaim or an overclaim, both of which can trigger a review. |
| 7 | Prior period corrections | If you identify an error from a previous quarter, do not simply adjust the current BAS without noting it. Errors above the ATO’s amendment threshold require a separate amendment request. | Rolling corrections into the current BAS without disclosure is treated as a new error, not a correction. The ATO’s amendment threshold for GST is $10,000 net GST or 1% of your annual GST turnover, whichever is lesser. |
After you complete the checklist
Once all seven checks are done, run a final comparison: your BAS GST figures against your accounting software’s GST summary report for the period 1 July to 30 September 2026. The two should reconcile to the cent. If they do not, there is an unresolved coding issue somewhere in your books that needs to be found before lodgement.
Key takeaway: The ATO cross-references your BAS against STP data, your prior lodgements, and third-party information notices. A BAS that reconciles internally but does not match those external data points will attract attention. Reconciliation is not optional — it is your first line of defence.
Why the Agent Deadline Is About More Than an Extra Month
The 25 November deadline for registered BAS agents is widely understood as a deadline extension. What is less understood is what a good BAS agent actually does with that extra time on your behalf.
What a registered BAS agent does that software cannot
Your accounting software can reconcile figures. It cannot interpret them. A registered BAS agent brings three things to a BAS review that automation does not:
- Compliance judgement: Knowing whether a transaction is correctly coded is not always obvious. Mixed-use assets, intercompany transactions, and industry-specific GST concessions (common in construction, hospitality, and professional services) require human interpretation of the GST Act, not just a bank feed rule.
- STP cross-referencing: An agent will pull your STP year-to-date figures and compare them directly to W1 and W2 before lodging. This step is rarely done by business owners reconciling their own BAS.
- Audit trail documentation: If the ATO does query your BAS, a registered agent maintains records of the review process. This documentation significantly reduces your exposure in a formal review.
The real cost of an amended return
An amended BAS is not simply a correction. It signals to the ATO that your original lodgement was inaccurate, which can prompt a review of prior periods. The ATO’s due dates page notes that failure-to-lodge penalties apply even when the tax owed is nil. Interest charges on late payments compound daily.
Getting the BAS right the first time is not just about avoiding a fine. It is about not inviting the ATO to look more closely at your business.
If your books are up to date and all seven checks above are clear, lodging your own BAS by 28 October is entirely achievable. If there are unresolved transactions, payroll discrepancies, or any uncertainty about your GST treatment, engaging a registered BAS agent for your SME lodgement before 28 October is the more prudent path. You keep the extended deadline, you get a professional review, and you lodge with confidence rather than crossed fingers.
Get Your Q1 BAS Right Before the Deadline
The Q1 2026-27 BAS covers 1 July to 30 September. Self-lodgers have until 28 October 2026. Businesses using a registered BAS agent have until 25 November 2026.
The seven checks in this guide address the errors that trigger ATO queries most often: uncoded transactions, STP mismatches, incorrect GST method, and undisclosed prior-period corrections. Running through them before you lodge takes time. Discovering them after the ATO does takes considerably more.
Balance My Books provides BAS review and lodgement services for Sydney SMEs, with fixed pricing and no surprises. If you want a registered agent to review your Q1 figures and lodge on your behalf, book a BAS review before 28 October to secure the extended deadline.
