How this usually works
Work backwards from what the ATO needs and what drives your numbers. If your accounts are fully reconciled and your payroll is correct, the BAS largely becomes a verification exercise rather than a scramble.
Decide whether you are lodging yourself or through a registered BAS agent. Agent review catches common mistakes and can unlock extended due dates under the ATO lodgment program.
Australian context to keep in view
- BAS reports GST and often PAYG withholding; it can also include PAYG income tax instalments and, for some, fuel tax credits or FBT instalments.
- Quarterly due dates (self-lodgement): 28 Oct, 28 Feb, 28 Apr, 28 Jul. Monthly BAS is due on the 21st of the following month. Agents may have later dates for some quarters.
- Cash vs accrual basis affects when GST is recognised; ensure your software setting matches your ATO registration.
- W1/W2 must agree to Single Touch Payroll (STP) totals; mismatches are a common audit trigger.
BAS preparation checklist (best-practice workflow)
- Reconcile all bank, credit card and loan accounts to statement dates covering the BAS period.
- Confirm GST settings: cash or accrual basis, reporting frequency, and registration effective dates.
- Review sales: GST on taxable sales only; mark exports and GST-free sales correctly; remove duplicates or timing errors.
- Review purchases: check no-GST items (ATO payments, bank fees, many government charges, wages), split private portions, and ensure tax invoices are on file for credits over $82.50 (incl. GST).
- Payroll cross-check: reconcile STP year-to-date to W1 (gross) and W2 (withheld); ensure super, leave and allowances are mapped correctly.
- PAYG instalments: confirm the current rate/amount and vary if business income has changed materially.
- Run BAS/GST reports: investigate uncoded transactions, suspense/clearing accounts and negative tax lines.
- Document adjustments: prior-period corrections, bad debt write-offs, private use and asset purchases.
- Final review: compare to prior periods for reasonableness; save a workpaper pack (reports, reconciliations, notes).
- Lodge and pay: lodge via Online services or through a registered BAS/tax agent; schedule payment to protect cash flow.
DIY vs BAS agent vs accountant
DIY
Best for simple, low-volume businesses with clean books, few adjustments and confidence with software.
BAS agent
Best for accurate GST/PAYG reporting, review of coding, clearing exceptions and timely lodgement. Often includes extended ATO due dates.
Tax accountant
Best when BAS decisions affect tax planning, structures, asset purchases, loans/dividends, or when you want integrated year-end and BAS support.
Commercial fit
Compare fixed fee vs hourly, turnaround times, and how they communicate during peak lodgement periods.
Software tips: Xero, MYOB, QuickBooks
- Xero: use the BAS Reconciliation report; check the GST Exceptions and Unreconciled transactions; ensure tracking categories don’t hide uncoded GST.
- MYOB: verify Tax/GST codes on both sides of spend/receive money; review the GST Summary and Payroll Activity vs Register reports.
- QuickBooks Online: run the GST Detail and Exception reports; confirm Banking feed duplicates aren’t coded twice.
- All systems: lock prior periods after lodging to prevent silent changes and keep a saved copy of each lodged BAS and workpapers.
Common BAS errors to avoid
- Claiming GST on no-GST items: ATO/ASIC fees, bank charges, wages, fines and many government charges.
- Incorrect GST-free or export treatment due to missing documentation.
- Payroll/STP not matching W1/W2, especially after backpay or corrections.
- Not splitting private use on vehicle, phone or home-office expenses.
- Posting ATO payments/refunds to GST accounts instead of the ATO clearing account.
- Forgetting to vary PAYG instalments when profits fall or rise.
Due dates and lodgement options
Quarterly self-lodgement due dates are 28 Oct (Q1), 28 Feb (Q2), 28 Apr (Q3) and 28 Jul (Q4). Monthly BAS is due on the 21st of the following month. Using a registered BAS agent or tax accountant may give you extra time under the ATO lodgment program and helps reduce interest/penalties from avoidable errors.
Best next steps
Decide your goal: a clean set of books, a lodged BAS, payroll alignment, or end-to-end bookkeeping and BAS support. Then choose the right path:
- BAS agent services for review, corrections and lodgement.
- Bookkeeping services for ongoing reconciliations and month-end control.
- Payroll services if W1/W2 and STP are causing issues.
- Small business accountant if you want integrated BAS and tax planning.
Frequently asked questions
What is the best way to prepare BAS?
Use a tight checklist: reconcile accounts, verify GST basis and codes, match payroll/STP to W1/W2, confirm PAYG instalments, run BAS reports, document adjustments and lodge via Online services or a registered BAS/tax agent.
What should I check before deciding how to prepare BAS?
Consider business structure, GST registration, reporting frequency, payroll complexity, software capability and whether you want compliance-only support or broader advisory input.
When is BAS due?
Quarterly: 28 Oct, 28 Feb, 28 Apr, 28 Jul for self-lodgers. Monthly: 21st of the following month. Agents on the ATO program may have later dates for some quarters.
Should I DIY or use a BAS agent?
DIY is fine for simple books with few adjustments. Use a registered BAS or tax agent if you have payroll, complex GST, prior-period fixes, overdue BAS, or you want a review and potential lodgement extensions.