The errors that get returns kicked back, the ones that cost real money, and the ATO data-matching most people don't realise is happening.
Most Australian tax returns are simple enough to do online via myGov in 30 minutes. Most are also wrong in at least one small way. The ATO's data-matching net catches more than people realise, and the errors that escape are usually the expensive ones - capital gains, rental income, deductions claimed without records. Here are the 10 mistakes that come up most often, ranked by how often they actually cost taxpayers money.
Bank interest, dividends, distributions from managed funds and ETFs, platform earnings (Stake, Pearler, etc.) all get reported to the ATO directly. If you forget to declare them, the data match will catch it - usually within a few months of lodging. Use the ATO's pre-fill in myGov as your starting point and cross-check against your own records, particularly for any accounts opened mid-year that might not appear yet.
The fixed-rate method (currently 67c per hour) seems generous but has specific record-keeping requirements. You need a representative diary of hours worked from home for the year, not a guess. The actual-cost method needs receipts and apportionment calculations for everything claimed. From 2026-27 the new $1,000 instant tax deduction simplifies many WFH claims to a no-receipts $1,000. Until then, the records requirement is real.
Selling shares triggers a CGT event. Common mistakes: forgetting the sale entirely, calculating the gain without the right cost base, applying the 50% discount before subtracting capital losses (it's the other way round), or missing brokerage and other costs that legitimately reduce the gain. Our pieces on the 50% CGT discount and cost base walk through the maths.
Renting out a room on Airbnb, hosting a short stay, even letting a friend pay rent on your spare room - all of it is rental income to the ATO. The Sharing Economy Reporting Regime means platforms report seller and host income directly. The deductions side still applies, but the income side has to be declared first.
Uber driving, food delivery, freelancing on Upwork, selling on Etsy, content creation income, tutoring on the side. If it's a business (not a hobby), it's all assessable. See our side hustle tax piece for how the ATO distinguishes the two.
Investment property owners often miss the capital works deduction (2.5% per year on the building cost). A quantity surveyor's depreciation schedule pays for itself many times over in the first year of claiming. Without one, most owners simply don't claim what they're entitled to.
The Low Income Tax Offset (LITO) is calculated automatically. SAPTO for seniors, the spouse super tax offset, private health insurance rebate, family tax benefits - most are calculated automatically too. But some, like the zone tax offset (for taxpayers in remote areas) or the medical expenses tax offset (where it still applies), require active claiming.
You can claim up to $300 in work-related expenses without receipts, but only if you can show how you spent the money. Beyond $300, written evidence is required. The ATO has consistently flagged blanket "I claimed $299 of work expenses" claims as a pattern they look at. From 2026-27 the $1,000 instant deduction option removes the need for records up to $1,000 - but it's one or the other, not both.
If you have two jobs, you can only claim the tax-free threshold from one of them. People who tick "Yes" to the tax-free threshold question on both employers' TFN declarations under-withhold during the year and end up with a tax bill at return time. Use our Tax Return Calculator to model what you'll owe.
HECS repayments are based on "repayment income" - your taxable income plus reportable fringe benefits plus exempt foreign income plus reportable super contributions. People often calculate based on salary alone, then are surprised by a larger HECS bill at tax time. The HECS Calculator shows you the actual repayment.
Disclaimer: This article provides general information about Australian tax return errors. It is not personal financial or tax advice. Individual circumstances vary. For specific guidance, refer to the ATO or consult a registered tax agent.