Capital Gains Tax Mistakes Australians Make and How to Avoid Them

Updated 19 July 2026 · Property and home loans

A Capital Gains Tax Calculator gives a fast answer, but the answer is only as good as what goes in. These are the mistakes we see most often with capital gains tax calculations in Australia, and how to avoid them.

1. Assuming prices always rise

Property growth varies by location and period. Plan for flat or falling prices too.

2. Borrowing the maximum

Lenders test you at a higher rate, but your budget still needs room for rate rises, repairs and life changes.

3. Forgetting purchase costs

Stamp duty, conveyancing, inspections and moving can add several percent to the price.

4. Comparing headline rates only

Fees, offset accounts and flexibility on extra repayments can matter as much as the rate.

5. Overlooking the detail that matters most here

Your main residence is usually exempt from CGT. Keep records of improvements, since they add to the cost base.

How the estimated CGT changes with sale price

A common mistake is getting one input slightly wrong. Here is what happens to the estimated CGT when “Sale price ($)” is off by up to 20% in either direction, with everything else held steady.

Sale price ($)	Estimated CGT
560,000	$7,800.00
630,000	$21,450.00
700,000	$35,100.00
770,000	$48,750.00
840,000	$62,400.00

At 560,000 the result is $7,800.00; at 840,000 it is $62,400.00. If a small change in this input moves the answer a lot, it is worth double-checking that figure before you rely on the result.

A quick sense check

Gain = sale price − selling costs − cost base. Individuals who held the asset over 12 months halve the gain. Tax = taxable gain × marginal rate. With typical inputs the calculator returns estimated CGT of $35,100.00. If your own result looks wildly different, check that each figure is in the right unit and period (weekly, monthly or yearly) before drawing conclusions.