The refund estimate myTax shows on-screen while you're lodging is calculated instantly from exactly what you've entered — the Notice of Assessment (NOA) that arrives afterwards reflects the ATO's own processing, cross-checks, and any debts or adjustments the on-screen estimate never knew about. A difference between the two numbers is common and doesn't necessarily mean either figure was wrong. This is one of the most common "is something wrong" questions AusTax AI hears during peak lodgment weeks.
Why would the ATO's final number differ from myTax's estimate?
The on-screen estimate is a live calculation: income tax, offsets, Medicare levy, and HECS-HELP repayment, based purely on the data sitting in the form at that moment. The NOA is issued after the ATO (ATO guidelines) has actually processed your return — checked it against employer, bank, and third-party data, applied any corrections, and settled anything else on your ATO account. Those are two different points in time, and a few things can change between them.
The most common reasons for a difference
| Reason | What happens |
|---|---|
| Data-matching correction | ATO's systems catch a mismatch against employer/bank/third-party data and adjust a figure |
| Offsetting other debts | Refund reduced to cover a debt to the ATO, Child Support, Centrelink, or another government agency |
| Employer finalisation change | Income statement updates after you lodged, changing the final figures |
| HECS-HELP reconciliation | Final repayment income calculated slightly differently once all data is in |
| Manual review adjustment | A flagged item is checked and corrected before the assessment issues |
In short, most gaps come down to the NOA reflecting a more complete, ATO-verified picture than whatever was on-screen when you clicked lodge.
What if the NOA is significantly lower than the estimate?
A meaningfully smaller refund is most often an offset against an existing debt — your own prior-year ATO debt, a Child Support Agency debt, or a debt to another Commonwealth agency the ATO collects on behalf of. The NOA itself usually shows this as a separate line, sometimes with a reference to the other agency, rather than folding it silently into the tax calculation. If your return is also flagged for extra checks rather than an offset, see our guide on what "under review" means and how long it takes.
What if the NOA is higher than the estimate?
This is less common but does happen — usually when the ATO's own data shows something in your favour that wasn't reflected on-screen, such as a corrected offset calculation or updated prefill data that arrived after you lodged. It's worth reading the NOA closely rather than assuming it's an error either way, since the line items explain exactly what changed. A higher-than-expected refund is not a reason to assume you can spend it immediately — if the ATO later identifies its own processing error, it can and will correct the assessment, which may mean repaying part of it.
How do I check what actually changed?
Your Notice of Assessment breaks down each component — taxable income, gross tax, offsets, Medicare levy, HECS-HELP, and credit for tax withheld — so you can compare it line by line against what you entered. Our guide on reading your Notice of Assessment walks through what each section means. AusTax AI's dashboard keeps a copy of what was actually submitted alongside the ATO's final figures once they're available, so the comparison doesn't rely on memory.
Quick checklist
- Compare the NOA line by line against what you entered, not just the bottom-line number
- Check for an "amount applied to other debt" or similar line before assuming a calculation error
- If your income statement changed after you lodged, that alone explains most differences
- Contact the ATO directly if a debt offset appears that you don't recognise
For the mechanics behind the original estimate, see our explainer on how the tax refund calculation actually works.
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*This is general information only, not personal tax advice. If your Notice of Assessment includes a debt offset or adjustment you don't understand, contact the ATO or a Registered Tax Agent for your specific situation.*