Quarterly BAS due 3 March: BAS checklist + GST mistakes to avoid

If you’re an Australian business owner, the quarterly Business Activity Statement (BAS) for the October–December period is due by 3 March. It’s crucial to prepare a clean, accurate BAS to meet this deadline and avoid errors that could cost you time or money.

In this guide, we provide a step-by-step BAS checklist and highlight common GST mistakes to avoid, helping you lodge your BAS smoothly and on time. By following these tips, you can minimise the risk of ATO penalties and avoid the rework that comes from correcting BAS errors.

BAS Due Date: 3 March 2026 and What It Means

For most small and medium businesses in Australia, the BAS is lodged quarterly. The BAS due on 3 March 2026 covers the period 1 October 2025 to 31 December 2025. This one-month extension beyond the quarter’s end accounts for the summer holiday period, so no further extensions apply for this quarter – even if you lodge online or use an agent, 3 March is the final date. Mark this date on your calendar and plan to lodge and pay on time. If the due date falls on a weekend or public holiday, the deadline shifts to the next business day.

Missing a BAS deadline can result in late lodgement penalties and interest charges, so timeliness is key. If you anticipate any trouble meeting the deadline, contact the ATO or your tax agent as early as possible to discuss extensions or payment plans.

Tip: Businesses using a registered BAS agent may receive a four-week lodgement extension for quarters 1, 3, and 4, but no agent extension is available for the December quarter BAS (since 3 March is already an extended date for everyone). So even with an accountant’s help, keep 3 March firmly in sight for your Q2 BAS.

Quarterly BAS checklist: A clean step by step workflow

This process is designed for SMEs using Xero, MYOB, QuickBooks, or similar cloud accounting. It reduces rework and helps your accountant or BAS agent lodge quickly.

1) Clean up GST coding (before you reconcile)

Most BAS issues start with incorrect GST treatment. The ATO’s own BAS and GST tips are a useful baseline for keeping GST classification consistent quarter to quarter.

Quick GST coding checks:

• Sales: taxable vs GST‑free vs input‑taxed are separated correctly.

• Purchases: GST credits are only claimed where GST is actually included and you’re entitled to claim.

• Mixed-use expenses: private portion is separated (especially vehicles, mobile, home internet, meals).

• Online subscriptions and software: check tax treatment (common miscodes).

• Large one-offs: assets/equipment, insurance annuals, fit‑outs — confirm coding and documentation.

If your business uses Simpler BAS reporting, the ATO’s Simpler BAS GST bookkeeping guide can help your team classify transactions correctly. Check the full guide here.

2) Reconcile bank accounts and credit cards (100% complete)

A BAS built on unreconciled accounts is a guess.

Do this every quarter:

• Reconcile every bank account and credit card used by the business.

• Clear “suspense”, “uncategorised”, and “clearing” accounts (payment gateways, undeposited funds, etc.).

• Make sure there are no unreconciled items parked for “later”.

Our  SMB services include financial reconciliations to keep general ledger accounts accurate and decision‑ready, explore the full details here.

3) Check sales vs deposits (prove revenue is right)

This is one of the fastest ways to spot GST errors.

Do a simple three-way check:

• Sales report (invoices/POS)

• Bank deposits (what hit the account)

• Income accounts (what’s recorded in the ledger)

Watch for these common traps:

• Customer deposits recorded as full sales too early

• Loans, transfers, owner contributions coded as income

• Payment gateway batching that doesn’t match invoice timing

4) Payroll alignment (wages and PAYG must match)

Even if your GST is perfect, payroll mismatches slow BAS finalisation.

Checks that prevent back‑and‑forth:

• Wages and PAYG withheld match your payroll reports for the quarter.

• Payroll clearing accounts (if used) reconcile to zero (or a known timing difference).

• Super accruals vs payments are understood (especially around quarter end).

Sageon provides comprehensive payroll services, including payroll processing and super guarantee lodgements, which helps keep BAS payroll labels aligned. 

5) Run the BAS “sanity test” (5 minutes)

 Before lodgement:

• Compare this quarter to last quarter (GST collected, GST credits, net amount). Big swings should have a clear reason.

• Review “adjustments” (credit notes, bad debts, mixed-use corrections).

• If you’ve made an error in a prior BAS, the ATO has specific guidance on how to correct GST errors in a later BAS (where eligible), get your full guidance here. 

GST mistakes to avoid before you lodge

These are the GST mistakes we see most often cause delays and rework (and they’re avoidable with the checklist above):

1. Claiming GST credits without proper support Keep GST records and documentation clean especially for higher-value purchases. 

2. Incorrectly treating GST‑free or input‑taxed items Misclassifying sales or purchases changes your GST position fast.

3. Coding private or owner transactions as business expenses This inflates GST credits and distorts reporting.

4. Missing adjustments Refunds, discounts, bad debts, and changes to consideration can require GST adjustments.

5. Not reconciling clearing accounts Payment gateways and undeposited funds create “phantom” GST problems if left unresolved.

   
   

If you want a simple reference point, the ATO’s BAS and GST tips page is a practical place to start for avoiding manual errors and getting BAS preparation right.

What to send Sageon for a fast, accurate BAS

To keep your BAS accurate (and minimise back‑and‑forth), send our team at Sageon the BAS pack below.

Your BAS pack:

• Confirmation that bank and credit cards are reconciled (or access so our team can reconcile)

• Draft BAS/GST reports from your accounting software (cash/accrual correctly selected)

• Sales summary (POS/invoiced sales) + notes on any unusual spikes/drops

• List of large or unusual transactions (assets, vehicles, insurance annuals, major software purchases)

• Payroll summary for the quarter (wages + PAYG withheld)

• Any ATO letters or messages received during the quarter

  
  

If anything’s missing, our team will tell you quickly. The aim is a clean BAS and a predictable process. 

How Sageon can manage BAS end-to-end

If BAS is taking too much headspace, Sageon can manage the full cycle. This includes clean up, reconciliation, reporting and lodgement as a part of our ongoing SMB support.

Here is what that looks like in practice.

1. Financial reconciliations so your BAS is built on solid numbers

2. Payroll support to keep wages and PAYG aligned each quarter

3. BAS lodgement covering GST, PAYG withholding, PAYG instalments, fuel tax credits and deferred GST where applicable

4. ATO liaison when needed, so you are not stuck in admin loops

If you also want to be more proactive, not just compliant, Sageon’s tax planning services include GST management. This can include GST registration, reviewing taxable supplies, and applying the GST margin scheme where relevant.

Get your quarterly BAS done once and done properly

A clean BAS is not about doing more work. It is about doing the right checks in the right order. GST coding, reconciliations, sales vs deposits, then payroll alignment. That is what prevents rework and keeps your cash flow clearer.

If you would like our team at Sageon to manage your BAS end to end from tidy up to lodgement, get in touch here.