The most expensive way to do BAS is to discover a coding problem on lodgement day. The cheapest way is to run the same disciplined pre-lodgement check on every client, every quarter, until it becomes the rhythm of the practice. This is the checklist we run with our customers; nothing in it is novel, but doing all five steps before pressing send is what separates a quiet lodgement from a phone-call afternoon.
All thresholds, dates and references below are cited to current ATO sources. Always confirm at ato.gov.au before relying on a specific figure — this is general guidance, not advice for a specific client situation.
The lodgement dates that actually apply
For quarterly BAS lodgers, the ATO sets out due dates on its Due dates for lodging and paying your BAS page:
- Quarter 1 (July–September) — due 28 October
- Quarter 2 (October–December) — due 28 February (extended to give visibility over the Christmas/January period)
- Quarter 3 (January–March) — due 28 April
- Quarter 4 (April–June) — due 28 July
If the due date falls on a weekend or public holiday, the next business day applies.
Practices lodging through a registered BAS or tax agent generally get a further four-week extension under the BAS agent lodgement program. The exact dates differ slightly each year — confirm the current program with the ATO before relying on the extension.
What the BAS actually contains (G-labels)
The BAS uses G-labels to drive the GST calculation. They split cleanly into sales and purchases, and the most common coding mistakes come from confusing which label a transaction should hit.
Sales side:
- G1 — Total sales (incl. GST)
- G2 — Export sales (GST-free)
- G3 — Other GST-free sales
- G4 — Input-taxed sales
- G5 = G2 + G3 + G4 (sales not contributing to GST)
- G6 = G1 − G5 (taxable sales, the figure that actually bears 10% GST)
- G7 — Adjustments
- G8 = G6 + G7
- G9 = G8 ÷ 11 (GST on sales — flows to 1A)
Purchases side:
- G10 — Capital purchases
- G11 — Other (non-capital) purchases
- G12 = G10 + G11
- G13 — Purchases for making input-taxed supplies
- G14 — Purchases without GST in the price (GST-free purchases)
- G15 — Estimated private use
- G16 = G13 + G14 + G15 (purchases not contributing to GST credits)
- G17 = G12 − G16
- G18 — Adjustments
- G19 = G17 + G18
- G20 = G19 ÷ 11 (GST credits — flows to 1B)
Net GST = 1A − 1B. (See the ATO's BAS completion guide for the worked examples.)
A common error: assuming G6 is a "GST-free purchases" line. It isn't — G6 is the taxable-sales calculation. GST-free purchases land at G14. Worth keeping this distinction handy when you're reviewing per-account contributions to the form.
The W1 / STP cross-check the ATO already runs
If your client has employees, W1 (Total salary, wages and other payments) on the BAS is also reported through Single Touch Payroll on a pay-by-pay basis. The ATO holds both numbers; a material mismatch generates a query.
The mechanical fix is simple but easy to get wrong: pay runs need to be journalled by the period they cover, not the date they were processed. A pay run for the last week of September that gets processed on 2 October should still hit the September quarter for both BAS W1 and STP purposes. Most modern payroll systems handle this correctly when configured; manual journal corrections are where the drift creeps in.
Before lodging the BAS, pull the STP YTD report from the payroll platform and compare the quarter's total reported wages against W1. If they diverge, fix at the source (payroll) before lodging the BAS, not the other way around.
The five-step pre-lodgement audit
For each client, before clicking lodge:
1. Reconcile every bank account to the source statement
Every transaction in the period needs to match a real bank-statement line. Unreconciled balances at quarter end are the single most common cause of post-lodgement corrections. ReconLink, MYOB, Xero and Reckon all surface reconciliation differences in the same way — chase any non-zero variance before the audit, not after.
2. Audit GST coding on top-vendor purchases
Pull the top 20 vendors by spend for the quarter. For each, confirm the coding is consistent with the previous quarter and matches the GST treatment expected for that vendor type:
- Telco, utilities, software (taxable) — GST code, full credit
- Bank fees, merchant fees (input-taxed financial supplies, per the ATO financial supplies page) — INP code, no GST credit
- Government charges, ATO payments, ASIC fees — N-T (out-of-scope), no impact on BAS
- Foreign software subscribers without GST on invoice — typically FRE or N-T depending on supply nature
The 80/20 of audit findings comes from the top-20 vendors. Spending more than 30 minutes on the long-tail vendors is usually wasted.
3. W1 / STP reconciliation
As above — pull the STP YTD wages for the quarter, confirm match with W1 within tolerance. Investigate any mismatch at the payroll source.
4. Capital purchases review
For any purchases that should be coded as capital (CAP code, hitting G10):
- Confirm the cost is correctly categorised against the instant asset write-off threshold. For the 2025–26 income year the threshold is $20,000 (assets first used or installed ready for use between 1 July 2025 and 30 June 2026, eligible business turnover under $10 million).
- For assets above the threshold, confirm they're going to the small business general pool, not being claimed in full.
CAP coding affects the BAS G-labels separately from the income-tax deduction timing — both need to be right.
5. Period close
Once the audit is clean, lock the period in your reconciliation tool. A back-dated transaction posted after lodgement is the most common cause of "the BAS we lodged doesn't match the books we're showing the client." Period-close discipline prevents this from happening silently.
A note on what's not in this checklist
Other things matter — PAYG instalments, fuel tax credits, wine equalisation tax, luxury car tax for relevant clients — but each is sufficiently client-specific that we'd rather not include generic guidance here. The ATO publishes detailed completion notes for every BAS label; for anything outside the GST/PAYG-W common path, refer to those before assuming a treatment.
This article was last reviewed against ATO guidance on the date shown in the frontmatter. BAS labels, thresholds and lodgement dates change — always confirm the current figures at ato.gov.au before relying on a specific number. This article is general guidance, not specific tax advice; for client-specific questions, consult a registered tax or BAS agent.