GST, BAS & the Tax Hand-off
A full health-check audit, a year of GST mis-coding fixed, a $13,183.50 mystery payment solved — and the most important lesson of all: the best move an AI made all day was to stop.
The golden rule, set up front
"Prior financial years are already lodged with the ATO — do not touch them." That single instruction shaped everything that followed. Watching Claude respect it — repeatedly handing back instead of guessing — is the best teaching moment in this whole case study. Set your guardrails before the agent starts, not after.
Fixed a year of GST mis-coding
Claude pulled the full GST detail and found GST being wrongly claimed on things that never carried Australian GST:
Counter-intuitive lesson: the corrections made me owe slightly more GST, not less. Claude was honest about that. It wasn't trying to "win" — it was trying to be right. (It also corrected my own misconception: keeping unpaid invoices on the books doesn't reduce tax — writing off genuinely bad debt does.)
A full health-check audit
Before changing anything, Claude ran a read-only audit and surfaced what actually mattered.
The whole current financial year was unreconciled.
A customer payment sitting unprocessed — QuickBooks wanted to miscode it as "Amazon sales."
Accounts receivable overstated across 72 invoices, most from 2016–2021.
A payroll clearing balance, unpaid super and PAYG, and a director's loan with possible Division 7A implications.
Solving the $13,183.50 mystery payment
My favourite part. Rather than brute-forcing it, Claude untangled a real sub-ledger error that had been quietly throwing the books out:
Read the remittance advice from my email
A PDF in Gmail showing exactly what the payment covered.
Discovered it covered five invoices
But two had already been wrongly marked paid in QuickBooks — the root of the confusion.
Recorded it correctly
Applied the payment against the genuinely-open invoices, booked the remainder as a customer credit, and matched it to the bank feed.
Interactive: Fix or Flag?
The whole skill of agentic bookkeeping is knowing what an AI should fix itself vs hand to a human. For each real situation from this case, decide: should Claude Fix it, or Flag it for the accountant?
The BAS question worth $7,000
Claude pulled the GST Amendment Report and found ~$6,940 of prior-period GST on sales (mostly AIM and Thomson Reuters invoices) being flagged — and a multi-year gap in QuickBooks' marked-as-lodged BAS history. It connected that straight to the current BAS:
What QuickBooks auto-pulled in
The rest is prior-period amounts
The real question it raised: have those prior-period amounts already been reported to the ATO? If so, paying this BAS as-is would be paying twice. Claude prepared the BAS so I could see the numbers — then stopped at the lodge step and handed the judgement to my accountant.
The accountant hand-off
Claude wrote a tight email to my accountant with five specific, dollar-tagged questions — turning a "please do my whole year" job into a short, cheap review. It drafted the email but didn't send it; I did.
The lessons worth teaching
The honest caveat — say this to your team
Claude is brilliant at the legwork and genuinely good at spotting things — but it is not a registered tax agent. Tax positions, Division 7A, whether prior-period GST was already lodged — those got handed to the accountant on purpose. The win isn't "AI does your tax unsupervised." It's AI does the 90% that's tedious and error-prone, surfaces the 10% that needs a human, and leaves a clean trail for both of you. That's the model worth teaching.