Capital gains tax catches a lot of Australians the wrong way. Not because the rules are uniquely complicated — they aren't — but because the most expensive mistakes hide in places people never think to look. Here are five CGT traps that have cost Australians a fortune, and exactly how each one bites.
1Missing the 12-month discount window
If you're an individual and you've held a CGT asset for more than 12 months before selling, only half of any capital gain is taxed. It's the 50% CGT discount — and it's worth a fortune. Sell at 12 months and a day, and you halve your tax bill. Sell at 11 months and 29 days, and you don't.
On a $100,000 gain, that timing difference is $50,000 of taxable income one way or the other. People miss it constantly — selling early because they spotted what looked like a market peak, or because they forgot when they bought.
The tricky bit is which dates the ATO actually uses. The 12 months runs from the contract date when you bought to the contract date when you sell — not the settlement dates. People watching the calendar against settlement can miss the window by weeks.
2Becoming a foreign resident on the family home
This trap has hit Australians overseas hard. Since 1 July 2020, if you're a foreign resident for tax purposes at the moment you sell your former Australian home, the main residence exemption is gone — even if you lived in the home for decades before leaving. Limited "life event" exceptions exist (death, terminal illness, divorce within six years of leaving) but they're narrow and easy to miss.
The bite is brutal. A home that would have been completely CGT-free can suddenly produce a six-figure tax bill, purely because the seller is on the wrong side of the residency line on contract day. There's no partial relief based on the years lived in the home as a resident; lose your eligibility and the whole gain is taxable.
We covered the exemption itself in the family-home tax shelter article.
3Letting the six-year rule expire
You can move out of your main residence, rent it out, and still keep it CGT-free for up to six years under what's known as the "absence rule". It's a powerful concession — but the clock matters.
Rent the home out for seven years before selling and the exemption is lost for the time beyond six years. Worse, the gain that gets taxed isn't just the bit after year six. The calculation uses the property's market value on the day you first started using it to produce income as the new cost base, so a sharp price rise during the rented years can produce a big taxable gain.
If you instead leave the home empty rather than rent it, the absence period can be indefinite — the six-year clock only applies when the property is generating income.
4Treating crypto like cash
Every disposal of a cryptoasset is a CGT event in Australia. Selling crypto for AUD, swapping one coin for another, paying for something with crypto, transferring it to someone as a gift — all of them count. The ATO is well aware: it data-matches Australian crypto exchanges directly to your tax file number and routinely sends "please explain" letters.
The trap is that active traders rack up hundreds or thousands of CGT events without realising. When the ATO comes calling, reconstructing two years of cross-exchange trades — including the AUD value at the exact moment of each trade — is a nightmare. Penalties for underdeclaring follow.
5Forgetting the costs that lower the gain
Your capital gain isn't just sale price minus purchase price. The cost base includes a long list of incidental costs people regularly forget — and every one of them reduces the gain that's taxed.
- Buying and selling agent fees and commissions
- Conveyancing and legal fees on the purchase and sale
- Stamp duty paid on the original purchase (for property)
- Brokerage on shares
- Cost of capital improvements (renovations, additions) on an investment property
- In some cases, interest on borrowings used to acquire the asset, and rates/insurance — for non-income-producing assets
A property with $30,000 of forgotten purchase and sale costs adds $30,000 to the taxable gain that shouldn't be there. At a 37% marginal tax rate that's more than $11,000 of unnecessary tax, or — for a long-held asset that gets the 50% discount — more than $5,500 still gone.
One more thing: the 2027 change is coming
Worth keeping in mind across all of these. From 1 July 2027, the 50% CGT discount is being replaced by cost-base indexation and a 30% minimum tax on gains, announced in the May 2026 federal budget. Gains accrued before 1 July 2027 on assets you already own will keep the current 50% discount; gains after fall under the new rules. For investors sitting on long-held assets with big embedded gains, the timing of a sale around mid-2027 will matter — and the family home stays fully exempt either way. We broke down the detail in our 2026 federal budget update.
The bottom line
None of these traps requires complicated tax structures to avoid. They require knowing the timing rules and finding the paperwork. A bit of attention before you sell — checking the contract date, your residency, your six-year clock, your crypto records, and every cost that lifts your cost base — is the difference between paying the CGT you actually owe and paying more than you owe.