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GST sanity check before lodging

Compare this quarter with last, and catch GST slip-ups before you or your BAS agent lodge.

Try it: 10 minIntermediateSample file included

Your BAS is only as good as the GST codes behind it. A quick side-by-side of this quarter and last quarter can show up a mis-coded sale, a missing tax invoice or a purchase counted twice. An AI agent can do the comparing quickly. It points at lines to look at. You check them, fix them in your own software, and lodge, or hand the numbers to your BAS agent. Quarterly BAS for July to September is due 28 October. You may get extra time if you lodge online or through a registered tax or BAS agent, so check the due date shown on your BAS.

The steps.

Step 1. From Xero, MYOB or QuickBooks, export your GST (or BAS) summary for this quarter and for last quarter, plus this quarter's transaction list showing the GST code on each line. Keep using the system you already have. Share exported files only, and never give an AI tool your bank or accounting login.

Step 2. Do a quick sense check yourself. The GST in a GST-inclusive price is one eleventh of it. If most of your sales are taxable, your GST on sales should be about one eleventh of your taxable sales including GST.

Step 3. Give your agent the exports and the prompt below. It compares the two quarters, flags big swings and lines that look wrong, and lists questions for your BAS agent. It doesn't work out your BAS and it doesn't change anything.

Step 4. Go through every flagged line in your own software. Some will be fine, because a big job really can cause a big swing. Others will be real slip-ups. The list below shows the usual ones.

Step 5. Fix what's wrong in your accounting software, then run the GST report again and check the totals moved the way you expected. If you can't fix something, note it for your BAS agent or accountant.

Step 6. Check your due date, then lodge, or send your BAS agent the summary, the transaction list and your questions. If you find a mistake in a BAS you've already lodged, the ATO says most GST mistakes can be corrected in your current BAS, so ask your BAS agent or see the ATO's guide to correcting GST errors.

Slips worth checking.

A purchase of more than $82.50 with no tax invoice. You generally need one to claim the GST credit, and the ATO says to wait until you have it.

A GST credit claimed where no GST was charged, such as bank fees, or on GST-free items like basic food.

A private purchase, or a part-private one. You can only claim the business part.

A taxable sale with no GST on it. Selling a business asset, like an old trailer or a trade-in, normally goes on your BAS as a sale with GST, so check it with your BAS agent.

Equipment coded as an everyday expense. Machinery, computers and cars are capital purchases. On a BAS that asks for them separately, they go at G10, apart from everyday expenses at G11, so ask your BAS agent how to code them.

The same invoice counted twice.

A sale or purchase in the wrong quarter, especially if you report GST on a cash basis, which goes by the date you paid or got paid.

Claiming a GST credit and also deducting the GST part of the same expense in your income tax return. Your deduction is the price less the GST credit.

A big swing you can't explain. It might be a real big job. It might be a wrong code.

A second pair of eyes in your accounting app.

Some accounting apps are adding their own GST checks. MYOB's AI BAS (a paid add-on, for simple cash-basis BAS) flags missing documents and unusual GST. QuickBooks has a GST AI Agent (Beta, with limited availability) that compares the income on your Profit and Loss with your BAS net sales before you file. Neither lodges your BAS for you. They're first drafts too, so you or your BAS agent still check before lodging. Nothing here means you need to switch apps.

Here is the prompt to copy and paste. Here is my GST summary for [this quarter, for example July to September 2026] and for [last quarter], plus my transaction list for [this quarter] with GST codes. I'm registered for GST in Australia and report on a [cash or accruals] basis. Don't work out my BAS, don't change anything and don't give tax advice. Please: 1) compare the two quarters and list any big swings in GST on sales, GST credits or total sales, with possible reasons for me to check; 2) flag lines where the GST code or GST amount looks wrong, for example GST claimed on bank fees, a sale with no GST, or a GST-free item coded as taxable; 3) flag purchases over $82.50 with no tax invoice or receipt noted; 4) flag anything that looks private, counted twice, or like equipment coded as an everyday expense; 5) end with a short list of questions for my BAS agent. Put every flagged line in a table with the date, description, amount and why you flagged it. Keep it in plain English.

Before you rely on it.

Your agent can miss things, and it can flag lines that are fine. Check every figure, GST code and date yourself. The ATO says it is seeing small businesses over-claim expenses and GST credits, so it pays to have a record behind each claim. If you're unsure about anything, ask the ATO or your BAS agent. This is general information, not advice.

A few minutes of checking now beats fixing a BAS later.

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