I've already lodged my tax return and found a mistake — how do I fix it?

You lodged in July to get the refund moving. Then in September a dividend statement turned up for shares you'd forgotten about, or your employer updated your income statement, or you found the receipt for a course you meant to claim. The return is already in. Now what?

The ATO has a standard process for exactly this, and it's less painful than most people expect. What matters is the order you do things in and how quickly you do them.

When an amendment is the right tool

The ATO's guidance (updated April 2026) lists the usual reasons: you answered a question incorrectly, left out income or a capital gain, didn't claim a deduction or offset you were entitled to, or something changed after you lodged — for example a revised or additional payment summary, your employer finalising or updating your income statement, or repaying income you were overpaid.

One exception first. If you think the ATO made the error when processing your return, contact the ATO; it says it may be able to sort it out without an amendment. And if you want to dispute the law or the facts the ATO used, or you're outside the time limit, the tool is an objection rather than an amendment.

How to lodge an amendment

Wait for the ATO's notification that your original return has been processed before lodging an amendment. The ATO says doing it earlier causes processing delays.

Then you have four routes. Online is the quickest: in myGov, open the ATO, choose 'Manage tax returns' and select 'Amend' next to the year; or in the ATO app, open your tax return and select 'Amend'. You can amend online however the original was lodged, and the ATO puts processing at about 20 days. You can overwrite a figure, add a new income or deduction type on the 'Personalise return' screen, or add another income statement, interest or dividend record on the 'Prepare return' screen.

The other routes are a registered tax agent (who can lodge the amendment electronically even if they didn't lodge the original), the paper form 'Request for amendment of income tax return for individuals', or a signed letter with the details the ATO specifies. Requests made in writing take up to 50 business days. There is no fee, and you don't send in a whole new return unless the ATO asks for one.

How long you have

Individuals generally have 2 years to amend an assessment, starting the day after the notice of assessment is sent. The ATO's own example: a myGov notice on 3 November 2026 means the period starts on 4 November 2026 and runs until 4 November 2028. For sole traders it is 2 years for 2023–24 and earlier years, and 4 years from 2024–25 onwards. You can lodge more than one amendment within that period.

The limit also binds the ATO: it generally can't amend your assessment more than 2 years after issue unless you ask it to — but that doesn't apply where there has been fraud or evasion.

If the fix means you owe more

An amendment that reduces your tax produces a refund (unless you have other tax debts). One that increases your tax is treated as a voluntary disclosure, and the online amendment, the paper form and a letter all count as the approved form.

The ATO's penalty guidance (updated March 2026) is where timing pays off. It won't apply a false or misleading statement penalty if you took reasonable care, even though the figure was wrong. Where a penalty does apply, it is reduced by 80% if you disclose before the ATO tells you it is examining your affairs — and if the shortfall is under $1,000, it is reduced to nil in that situation. Once an examination has been notified, the reduction drops to 20%, and only where your disclosure saved the ATO significant time or resources. Interest on the shortfall may also be reduced for an unprompted disclosure, though you may have to ask.

The two most common mistakes

The first is lodging a second tax return for the same year. That isn't how it works — the fix is an amendment to the existing assessment.

The second is waiting to see whether the ATO notices. Employers, banks, share registries and platforms report to the ATO, and its pre-fill and data matching exist to find gaps between what was reported and what you declared. If it gets there first, the most generous penalty reductions above are gone.

This is general information about ATO rules as at September 2026, not advice about your return. Whether a particular change needs an amendment — and what it does to your tax — depends on your circumstances.

Our individual tax returns service handles amendments as well as original returns, and our income tax calculator lets you check the effect of a changed figure first. For related reading, see our pieces on what pre-fill still misses and on the ATO's interest rates from 1 October 2026.