CPI Indexation vs the 50% CGT Discount: A Worked Example

The 50% CGT discount and CPI cost-base indexation both exist to stop tax hitting a gain that's partly just inflation. They don't produce the same tax bill. Here are two worked examples showing why, and when the new 30% minimum tax actually changes the answer.

The mechanics in short

Today (the 50% discount): hold an asset over 12 months, and only half the raw gain is taxable, added to your income at your marginal rate.

From 1 July 2027 (indexation + minimum tax): your cost base is inflated by the CPI movement between when you paid and when you sold, and the whole indexed gain is taxable at your marginal rate — with a 30% minimum tax top-up if your marginal rate on the gain would otherwise come in under 30%.

To keep the numbers clean, both examples below are for an asset bought and sold entirely after 1 July 2027 — no transition-date apportionment involved. (An asset that spans the transition uses a different mechanism — a deemed disposal and reacquisition at market value just before 1 July 2027 — covered in the 2027 Tax Changes Explained overview and calculated for real holdings by the CGT Calculator 2027 New Rules.)

Example 1: a taxpayer already at 39%

Priya buys shares for $50,000. Years later she sells for $90,000 — a raw gain of $40,000. Say cumulative inflation between purchase and sale was 15% (an assumption for this example — the real figure comes from the published ABS CPI series, which the calculator tool applies for you).

Under today's discount, if this sale happened before 1 July 2027: taxable gain = $40,000 × 50% = $20,000. At Priya's 37% marginal rate plus the 2% Medicare levy (39% combined): tax = $7,800.

Under indexation, after 1 July 2027: indexed cost base = $50,000 × 1.15 = $57,500. Indexed (taxable) gain = $90,000 − $57,500 = $32,500 — no 50% reduction applies. Ordinary tax at 39% = $12,675.

Checking the minimum tax: 30% of $32,500 is $9,750, which is less than the $12,675 Priya's marginal rate already produces — so the top-up is nil. She pays $12,675 either way under the new rules.

The difference: $4,875 more tax on an identical sale, purely from indexation (15% cumulative inflation) being worth less than the 50% discount was on this particular gain and holding period.

Example 2: a taxpayer at a lower marginal rate

Same $32,500 indexed gain, but now assume the seller's marginal rate on that gain is only 19% — someone with modest other income.

Ordinary tax at 19%: $32,500 × 19% = $6,175.

Now the minimum tax bites. Following the Act's method statement: 30% of the gain is $9,750; ordinary tax attributable to the gain is $6,175; the gap is $9,750 − $6,175 = $3,575 — payable as a top-up, on top of the ordinary assessment.

Total tax: $6,175 + $3,575 = $9,750 — exactly 30% of the gain. This is the mechanic working as designed: a floor of 30% on the effective rate paid on a capital gain specifically, for anyone whose marginal rate alone would have taxed it more lightly.

The takeaway

Indexation and the minimum tax pull in different directions depending on who you are:

• Higher-rate taxpayers (marginal rate at or above 30%) never pay the top-up — their result depends entirely on how indexation compares to what the 50% discount would have sheltered, which depends on inflation over the specific holding period.
• Lower-rate taxpayers (marginal rate below 30%) get their gain taxed at a 30% floor regardless — a genuine increase for anyone who previously relied on a low marginal rate plus the 50% discount to pay well under 30% overall.

Run your own numbers on the CGT Calculator 2027 New Rules — it shows the old-law and new-law figures side by side, with the indexation and minimum-tax steps broken out.