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): if you held the asset for at least 12 months and were not a foreign or temporary resident during the relevant period, 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 marginal rates. The top-up is the amount by which 30% of the defined minimum tax capital gain exceeds the extra basic income tax that gain adds, subject to statutory gain exclusions and the income-support exemption.

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 extra basic income tax the gain adds — so the top-up is nil. Including the assumed Medicare levy, she pays $12,675 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 adding it increases the seller's basic income tax by 19% across the whole gain — 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.

Basic income tax plus top-up: $6,175 + $3,575 = $9,750 — exactly 30% of the gain before offsets. This is the mechanic working as designed for this example: the top-up fills the gap between the basic income tax attributable to the defined gain and the statutory floor.

The takeaway

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

• The top marginal bracket is not the test. A gain that spans lower brackets can attract a top-up even if the taxpayer's highest rate is 30% or more.
• The defined gain and exemptions matter. Statutory gain exclusions and the income-support exemption can remove the top-up.

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.