BAS Quarter-End GST Checklist:
14 Days from Chaos to Lodged

What the 28th Actually Means — and Why Two Weeks Matter

The 28th of the month following quarter-end — 28 October, 28 February, 28 April, 28 July — is the date the ATO expects your BAS lodged and paid. But calling it a "deadline" flattens what's actually a sequence of staggered due dates depending on how you lodge.

Self-lodgers who file online through myGov or SBR-enabled accounting software get an extra two weeks on top of the 28th. Registered BAS agents get four additional weeks on Q1 and Q3, two extra weeks on Q4 — and nothing extra on Q2, because February already carries a one-month extension baked into the December quarter deadline.

These extensions matter strategically. If your accountant lodges, you have breathing room. If you lodge yourself, the 28th is the real date — the online two-week concession is a buffer, not a plan. Either way, the internal preparation timeline doesn't move: the quarter ends, the clock starts, and every day spent chasing missing invoices or re-coding transactions pushes your margin for error toward zero.

The ATO issues Failure to Lodge (FTL) penalties at $313 per penalty unit for each 28-day period a BAS is overdue, capped at five units — or $1,565 maximum per BAS for small entities with turnover under $1 million (ATO BAS agent lodgment program 2026–27). On top of that, the General Interest Charge (GIC) compounds daily at approximately 10.96% per annum on any unpaid GST or PAYG. You don't need to have underpaid to trigger an FTL penalty — lodging late, even if the ATO owes you a refund, still counts.

The 14-day countdown that follows assumes you're lodging yourself, the standard 28th is your drop-dead date, and every day you lose to manual reconciliation is a day you can't get back.

7 Days to Deadline: Close the Bank Accounts

Bank reconciliation is not step one because it's the most important — it's step one because every subsequent check depends on it. If your bank accounts aren't reconciled to the cent, your GST figures are built on incomplete data. A single missing deposit inflates your GST-on-sales liability. A single uncoded withdrawal hides a GST credit you could have claimed.

Start with the quarter's final week. Payments made on the last day of the quarter might not clear until the 2nd or 3rd of the following month — but they belong to the quarter just ended. Reconcile every linked bank and credit card account to the statement closing balance. If you use Xero, MYOB, or QuickBooks Online, confirm the Bank Reconciliation Summary report matches each statement exactly. Xero's reconciliation engine automates much of the matching, but automated matching isn't audit-proof — a payment that hits the wrong contact's invoice is technically reconciled and technically wrong.

Do not generate your BAS summary report until bank reconciliation is complete. Unreconciled transactions are excluded from the GST calculation entirely. If your bookkeeper only reconciles quarterly at BAS time, this is where the backlog hits hardest — and where duplicated supplier bills, missed invoices, and stale uncleared items surface simultaneously (the time cost of catching up quarterly is one reason manual BAS prep burns more hours than most owners budget for).

5 Days to Deadline: Lock Sales and Verify GST Codes

With bank accounts reconciled, you now have a complete picture of every transaction in the quarter. The next 48 hours are about GST classification — which, in BAS preparation, is where most errors originate.

Go through every sales transaction and confirm the GST treatment. The checkpoints are specific:

• Taxable sales: GST collected at 10%. These populate label G1 (Total sales) and 1A (GST on sales).
• GST-free sales: Exports, basic food, health services, education. These go into G1 but not into 1A. On a Full BAS (turnover $10 million or more), they also populate G2 (Export sales) or G3 (Other GST-free sales). On a Simpler BAS (turnover under $10 million), you don't report G2 or G3 at all — only G1, 1A, and 1B.
• Input-taxed sales: Residential rent, financial supplies. Included in G1 for reporting but no GST is charged, so no 1A impact.

On the expense side, the rule is: no valid tax invoice, no GST credit. For any purchase over $82.50, you need a tax invoice — not a receipt, not a bank statement line. A receipt shows you paid; a tax invoice shows the supplier's ABN, the GST amount, and the words "Tax Invoice." Without it, label 1B (GST on purchases) is overstated and the ATO can reverse the credit on audit.

If your business reports under the Full BAS method (turnover ≥$10M), the additional labels — G10 Capital purchases and G11 Non-capital purchases — must be reconciled against your fixed asset register and general ledger purchase accounts respectively. Simpler BAS filers skip these, but the underlying bookkeeping still has to be right: just because you're not reporting G10 to the ATO doesn't mean you can misclassify a $15,000 equipment purchase as an operating expense.

3 Days to Deadline: Cross-Check PAYG Withholding

W1 and W2 are the two PAYG withholding labels on your BAS. W1 is total gross wages, salaries, director fees, and other payments from which you withheld tax. W2 is the total tax you actually withheld from those payments. The ATO cross-references both against your Single Touch Payroll (STP) reports — and they should match to the cent.

Before you enter W1 and W2, confirm: all pay runs for the quarter have been finalised and reported through STP; superannuation contributions for the quarter have been paid or scheduled (SG is not a BAS item, but missed SG triggers the Super Guarantee Charge, which compounds into a separate ATO liability); and any allowances, bonuses, termination payments, or back-pay are included in W1. Exclude super contributions, salary sacrifice amounts, and payments where you withheld because an ABN wasn't quoted — those go to W4, not W1.

For bookkeepers managing multiple clients, the PAYG cross-check is the quietest source of BAS amendments. A payroll that closed on the 29th of the quarter with one extra pay run that the BAS draft missed — that's a W1 understatement and a reconciliation that fails ATO matching. The PAYG summary July deadline checklist covers the year-end version of this same reconciliation, but the quarterly discipline is the same: payroll report → BAS label → match.

1 Day Before Lodgement: The Full BAS Cross-Check

The day before you lodge is verification day. You're not entering new data — you're comparing what your software calculated against what your ledger says should be there. Every figure in your BAS draft should have a matching figure in a report you can print and file.

If you use Xero, run the Activity Statement report and the GST Reconciliation report side by side. 1A and 1B on the Activity Statement should match the GST collected and GST paid accounts in the reconciliation. If they don't, trace the variance through the GST Audit Report — a single miscoded transaction (GST applied to a GST-free purchase, or vice versa) is usually the culprit.

If you're on Simpler BAS, your verification is three fields: G1, 1A, 1B. If you're on Full BAS, you're verifying seven: G1, G2, G3, G10, G11, 1A, 1B — plus the net GST amount (1A minus 1B) which appears at label 1H. For PAYG instalments, confirm whether you're on the rate method (T1 × instalment rate) or the GDP-adjusted notional amount — and if your income has shifted more than 15% from the previous year, you can vary the instalment at label T4 (ATO GST reporting methods). Varying downward when your actual income is higher triggers interest on the shortfall, so be certain before you change it.

One final sanity check: compare this quarter's BAS to the same quarter last year. If Q3 2025 showed $22,000 in GST on sales and Q3 2026 shows $68,000 — and you didn't double your business — something is miscoded. The ATO's data-matching flags are trained on exactly these anomalies. A quick year-over-year comparison catches them before the lodgement button does.

Lodgement Day: What to Do Before You Hit Submit

If you lodge through a registered BAS agent, your deadline is extended — 25 November for Q1, 26 May for Q3, 25 August for Q4 — but your agent needs your reconciled figures well before those dates. Agent deadlines are delivery dates for the agent, not start dates for you. The internal timeline still runs on the same 14-day cadence; it just ends with sending a file to your agent instead of clicking "lodge" yourself.

If you lodge yourself through Xero, MYOB, or the ATO Business Portal, confirm the BAS period displayed on the lodgement screen matches the quarter you intend to lodge. It sounds obvious, but selecting the wrong period — especially when you're catching up on overdue BAS — is a common mistake that generates an ATO mismatch letter six weeks later.

Even if you had zero business activity in the quarter, you still lodge a nil BAS. Failing to lodge nil returns triggers the same FTL penalties as a BAS with amounts owing. In the ATO portal, select "Prepare as nil" rather than leaving fields blank.

If you owe money and can't pay in full by the due date, lodge on time anyway and contact the ATO before the due date to arrange a payment plan. Lodging late because you can't pay adds an FTL penalty on top of the GIC — two separate charges that don't offset each other. The ATO is more accommodating of payment plans when you've lodged on time and initiated contact proactively (ATO: How to lodge your BAS).

What Happens If You Miss the 28th

Day 1 after the deadline: The ATO's systems flag the overdue BAS. No penalty issues immediately — the system runs batch processing, and most first-time late lodgers receive a reminder before a formal penalty notice. But the General Interest Charge starts accruing from day one on any unpaid amount.

Day 28: The first FTL penalty unit ($313 for small entities) is applied. If you still haven't lodged, the counter resets — another 28 days, another penalty unit, up to the $1,565 cap. Medium entities ($1M–$20M turnover) pay double; large entities pay five times. These penalties are per BAS, so four late quarterly BAS lodgements can stack to $6,260 for a small business even if every one of them shows a refund position.

If BAS lodgement keeps slipping because the data-gathering phase eats too many hours, the bottleneck isn't the lodgement step — it's the extraction step. When you're manually pulling figures from invoices, receipts, and bank statements into a spreadsheet, a two-week window can compress into a weekend panic. The BAS data extraction guide walks through how to close that gap, and batch-processing four quarters into a single tax ledger turns a quarterly scramble into an annual rhythm. The same seasonal discipline applies whether you're facing the UK SA100 January deadline or an Australian BAS quarter-end — the countdown structure works, but only if the data is already digitised before the last week begins.

Frequently Asked Questions

What happens if the 28th falls on a weekend or public holiday?

The due date moves to the next business day. But this only helps if you're lodging electronically — paper lodgers need to account for postal delivery time. The public holiday that matters is the one observed where the ATO processes lodgements (typically ACT public holidays for electronic lodgement), not necessarily the holiday in your state.

How do I know if I should use Simpler BAS or Full BAS?

If your GST turnover is under $10 million and you haven't been directed otherwise by the ATO, you're on Simpler BAS by default — reporting only G1, 1A, and 1B for GST. If your turnover is $10 million or more, you must use the Full reporting method with all seven GST labels. The ATO determines your method based on your reported turnover, not your preference. If you cross the $10 million threshold mid-year, your reporting method changes from the start of the next GST reporting period following notification.

Can I switch from quarterly to monthly BAS reporting?

Yes, by contacting the ATO. Businesses with GST turnover of $20 million or more must report monthly; smaller businesses can request the change voluntarily. Monthly reporting means higher lodgement frequency but smaller payment amounts per cycle — some businesses prefer it for cash flow predictability. The switch takes effect from the start of a reporting period; contact the ATO before that period begins.

What if my W1 and W2 don't match my STP reports?

Don't lodge. A mismatch between BAS W1/W2 and STP-reported figures is one of the fastest triggers for an ATO data-matching letter. Trace the variance: check for pay runs that straddled the quarter boundary, employees with multiple pay rates that caused rounding discrepancies, or a pay run finalised after the quarter closed but dated within it. In Xero or MYOB, generate a payroll activity summary filtered to the exact BAS date range and compare line by line. If the discrepancy can't be resolved before the deadline, lodge the BAS with the figures you can verify and file an amendment once the payroll data is corrected — but know that an amendment triggers its own ATO review flag.

Can AI extraction help with BAS data gathering?

BAS preparation runs on documents: supplier invoices, purchase receipts, bank statements, payroll summaries. Each one holds line items that need to land in the right BAS label — but getting them there means opening each file, reading each figure, and manually entering it into your accounting software. This is where Custom Column Extraction changes the timeline: instead of reading documents and typing data, you define the output columns you need — "Supplier Name," "Invoice Date," "GST Amount," "Total" — and the AI locates each value across any document format, without templates or per-supplier setup. The manual lodgement burden isn't the form itself — it's the hours before the form, spent gathering what the form asks for.

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

Yes. A nil BAS must be lodged by the due date even if you made no sales, paid no wages, and collected no GST. Failing to lodge a nil BAS incurs the same FTL penalties as a BAS with amounts owing. In the ATO portal, select "Prepare as nil" — this populates the required fields with zeros and submits a compliant lodgement.