By Joshua Occhino, Co-founder, Ezyiah
Short answer: A BAS audit trail is defensible when you can show, for any figure on the BAS, the source transactions behind it, the GST treatment applied and why, who reviewed it, and what was changed and when. The ATO expects records that explain your transactions and are kept for five years. Balanced numbers alone aren't enough. A file can reconcile and still be hard to defend if the reasoning behind its adjustments has been lost.
What records does the ATO expect behind a BAS?
The ATO's record-keeping rules for business require records that explain all transactions, kept in English (or easily converted), generally for five years. For GST, that includes tax invoices when you claim a credit for a purchase over $82.50 (including GST). The ATO also explains accountability for record keeping: if you use an agent or software, you're still responsible for your records.
Why is "correct GST" not the same as "defensible GST"?
A GST figure can be correct and still hard to defend. Defensible means you can explain how you got there, consistently, across periods. Two businesses can report the same 1B figure. One can show the tax invoices, the coding decision for each mixed-use expense and the reason for a one-off adjustment. The other has the same total and no explanation. Under review, the first is resolved in one exchange. The second has to rebuild its reasoning months later.
Consistency matters as much as accuracy. GST treatment that changes between quarters without a stated reason invites questions even when each period is defensible on its own. See GST codes changing over time.
What makes a trail defensible?
- Traceability. Every BAS label can be traced back to transactions, and every transaction to a bank line or source document.
- Sequence. It's clear what happened first: import, coding, review, adjustment, lodgement.
- Recorded decisions. Adjustments carry a reason, especially for private use, director loans, mixed-use expenses and GST on imports.
- Review evidence. Someone reviewed the period, and you can tell who and when.
- Corrections handled properly. Errors are fixed in a later BAS or by revision under the ATO rules, not by adjusting the final number. See how long you can amend a BAS.
Why does this matter if the ATO asks questions?
If a shortfall is found, penalties depend on the behaviour behind it. A voluntary disclosure made before the ATO tells you about an examination can reduce the base penalty by 80%, and shortfalls under $1,000 may attract no penalty. A clear trail is what lets you find and disclose an error quickly, and show that reasonable care was taken. See what voluntary BAS disclosure does to your penalty.
How Ezyiah helps you keep the trail
Ezyiah is an AI bookkeeper with its own double-entry ledger. Transactions come in from live bank feeds or uploaded statements and are reconciled with AI. Receipts, bills and invoices are matched to the transactions they belong to. The BAS, General Ledger and Trial Balance build from the same books, so each figure traces back to its source. The Ezyiah Assistant panel and activity logs show what changed and when. Your accountant reviews and approves the BAS before lodgement.
See how it works for accounting firms and business owners.
Frequently asked questions
How long do I need to keep BAS records?
Generally five years from when you prepared or obtained them, or from when the transaction was completed, whichever is later. Check the ATO record-keeping rules for your situation.
Do I need a tax invoice to claim every GST credit?
You need a tax invoice to claim a GST credit for a purchase that costs more than $82.50 (including GST). For smaller purchases, other records such as a receipt are enough. See when you can claim a GST credit.
Is a balanced reconciliation enough evidence?
No. It shows the ledger matches the bank. It doesn't show that GST treatment is right or why adjustments were made.
