Correct an Invoice That Charged GST by Mistake

If you are not registered for GST, you should not charge GST on an ordinary invoice. When a GST line appears by mistake, the practical fix is to verify the facts, issue corrected paperwork, tell the customer the current balance clearly, and keep records showing what changed. This article is about the invoice correction process, not BAS or tax return adjustments. If the timing or registration facts are unclear, check with your accountant before resending anything.

Confirm the mistake before changing the invoice

Start with one question: were you actually unregistered for GST on the invoice date? An ABN and GST registration are different. Having an ABN does not, by itself, mean you can charge GST.

The ATO guidance also distinguishes GST registration from concepts people often mix together. GST turnover is not profit, and some businesses must register regardless of turnover, including taxi and ride-sourcing providers. If you are uncertain about whether registration was required at the time, stop and verify first using the ATO guidance on when GST registration is needed.

It also helps to separate two ideas that are not the same:

Concept	What it means here
Supplier not registered for GST	The supplier should not charge GST on the invoice
GST-free supply	A specific tax treatment; not the same thing as being unregistered

Before you edit anything, confirm:

the original invoice date
your GST registration status on that date
whether the customer has paid nothing, part, or all of the invoice
whether the quote, contract, or purchase order described the price as GST-inclusive or GST-exclusive

If those facts are messy, get advice before issuing a replacement. It is easier to pause than to correct the same invoice twice.

What the corrected invoice should show

For an unregistered supplier, label the corrected document Invoice, remove the GST amount and show the revised total clearly. Avoid presenting the document as though GST applies when it does not.

Australian invoicing guidance explains the standard information invoices commonly need, while the ATO explains what a tax invoice must contain when GST applies. See the Australian government invoicing guidance and the ATO tax-invoice guidance.

A practical format is to rebuild the document using a no GST invoice template for Australian businesses and add a direct note such as:

Supplier not registered for GST. No GST has been charged.

That wording tells the customer why the total changed. It also reduces the chance that someone assumes the work itself was altered, rather than the tax line being removed.

Scenario 1: the invoice is unpaid

If the customer has not paid yet, the cleanest approach is usually to keep the original in your records, issue a corrected replacement invoice, and tell the customer not to pay the earlier version.

Here is a fictional example in AUD.

Item	Original invoice	Corrected invoice
Design work	A$800.00	A$800.00
GST line	A$80.00	A$0.00
Total due	A$880.00	A$800.00
Amount paid	A$0.00	A$0.00
Current balance	A$880.00	A$800.00

Assumptions: the supplier was not GST-registered on the invoice date, and A$800.00 was always the intended charge for the service.

What to send is simple:

Send	Why it matters
Corrected invoice PDF	Gives the customer the amount that should now be paid
Short message withdrawing the earlier invoice	Prevents payment against the wrong version
Cross-reference to old and new invoice numbers	Makes the file easier to follow later
Exact current balance	Reduces duplicate payment mistakes

Useful wording:

Please disregard invoice INV-104 as issued in error. We are not registered for GST, so the GST line has been removed. The corrected invoice is attached. The current amount due is A$800.00. Please pay only the corrected balance.

The key is not just sending a revised PDF. You also need to say plainly which invoice should be ignored and what amount is now payable.

Scenario 2: the invoice has already been paid

If the customer already paid the incorrect total, your job is to document the corrected position and explain the overpayment clearly. Do not assume your invoicing software creates a credit note automatically; check what record you need to issue and keep.

A simple fictional example:

Item	Original invoice	Correct position after correction
Consulting services	A$1,200.00	A$1,200.00
GST line charged by mistake	A$120.00	A$0.00
Total invoiced	A$1,320.00	A$1,200.00
Amount customer paid	A$1,320.00	A$1,320.00
Overpayment to address		A$120.00

Assumptions: the supplier was not GST-registered on the invoice date and the customer paid in full.

In this paid scenario, send the corrected invoice or other clear correction document, confirm the correct service amount, state the overpayment, and set out the next agreed step.

Useful wording:

We identified that GST was added in error to invoice INV-221. We are not registered for GST, so the correct total for that invoice is A$1,200.00, not A$1,320.00. Our records show your payment of A$1,320.00 has been received. This leaves an overpayment of A$120.00. Please reply confirming whether you would prefer a refund of A$120.00 or, if appropriate, application to a future invoice.

That kind of message removes ambiguity. The customer should not be left guessing whether they still owe money, whether they are due a refund, or whether the old invoice still stands.

Keep a paper trail that still makes sense later

Even a small GST error should have a tidy audit trail. Months later, you or your accountant may need to reconstruct what happened.

Keep:

Record	Purpose
Original invoice	Shows the initial error
Corrected invoice or correction document	Shows the final billing position
Customer message	Proves what you told them and when
Any customer reply	Confirms receipt or agreement
Payment evidence	Confirms whether the invoice was unpaid, part-paid, or paid in full
Brief internal note	Explains why the correction was made

If you need a fresh document layout, use an Australian invoice without GST template. If you want broader guidance on how a non-GST invoice should look from the start, see How to Write an Australian Invoice Without GST.

Two edge cases that often cause confusion

The first is part payment. If the customer paid only part of the wrong total, do not estimate from memory. Check the actual payment record, then calculate the remaining balance from the corrected invoice total.

Example:

original mistaken total: A$550.00, including A$50.00 GST charged by mistake
corrected total: A$500.00
payment already received: A$300.00
corrected balance still due: A$200.00

Your message should say that A$300.00 has been received and the remaining balance is A$200.00. That avoids asking the customer to pay an amount they have already covered.

The second is when the customer asks you to leave the GST line in place because their system expects it. Do not do that. A buyer's internal workflow does not change your actual GST position. If you are not registered, the correction should reflect reality. You can add clarifying notes, but not a GST charge that should not exist.

A related complication is unclear contract wording. If a quote said only “A$1,100 including GST” and you were never registered, the commercial price may need interpretation. That is a good point to check with your accountant before deciding whether the intended fee was A$1,100 total or A$1,000 plus an incorrectly added amount.

Keep this correction separate from broader GST questions

This article answers a narrow question: how to correct an invoice that charged GST by mistake when the supplier was not registered. It is different from deciding whether you need GST registration for future invoices, and different again from drafting a clean non-GST invoice from scratch.

If your real question is about registration itself, read Do I Charge GST Under $75,000? The Threshold Explained. If you simply need the right format for a non-GST invoice, start with the no GST invoice template or read How to Write an Australian Invoice Without GST.

In practice, the best correction is usually straightforward: verify your status on the invoice date, remove the GST line, state the correct balance in one sentence, and preserve the records. For an unpaid invoice, that means replacing the amount due and telling the customer not to pay the old version. For a paid invoice, it means documenting the corrected total and any overpayment so nobody pays twice and the file stays clear.