These adjustments are reported on the AMMA statement issued by ETF providers each year. They do not create immediate tax, but they affect how much capital gain (or loss) will be calculated when you eventually sell. Understanding how they work is part of accurate record keeping.
Key takeaway
When an ETF distribution includes tax-deferred or return of capital amounts, your cost base may need to be adjusted. From 1 July 2027, eligible post-reform gains use indexation, but annual AMMA adjustments and parcel-level records still feed the calculation.
Law update (15 July 2026): the Treasury Laws Amendment (Tax Reform No. 1) Act 2026 is now law. For eligible Australian resident individuals and trusts, gains accruing from 1 July 2027 generally move from the 50% discount to an inflation-indexed cost base. Existing ETF parcels held across 30 June 2027 are generally split into deferred pre-reform and post-reform components. Your AMMA adjustments do not disappear; they remain part of the parcel-level records supporting those calculations.⁴
This is general educational content, not personal financial advice or tax advice. Tax situations vary. Consult a registered tax agent for guidance specific to your circumstances.
What a Cost Base Is#
Your cost base is the amount used to calculate capital gain or loss when you sell an investment. For shares and ETF units, it begins as the purchase price plus acquisition costs (such as brokerage).
Example: You buy 500 ETF units at $20.00 each and pay $10 in brokerage. Your cost base is:
(500 × $20.00) + $10 = $10,010.
When you eventually sell, your capital gain or loss is calculated as:
Sale proceeds − Cost base = Capital gain (or loss)
If your cost base changes between purchase and sale, the gain or loss calculation changes accordingly.
What the 2027 CGT Reform Adds#
For an eligible parcel held across the transition, the Act generally treats the asset as sold just before 1 July 2027 and reacquired immediately afterward at market value. The pre-reform gain or loss is deferred until the parcel is actually sold. The post-reform gain or loss is then calculated from the new transition cost base, with eligible cost-base elements indexed for inflation after 1 July 2027. An approved apportionment method may be available instead where its requirements are met.⁴
This does not replace the annual AMMA process. It adds a transition layer. For each parcel, preserve:
- the original acquisition cost and date
- every AMMA cost-base increase or decrease up to 30 June 2027
- evidence of the 30 June 2027 market value or the approved apportionment method used
- every AMMA adjustment after 1 July 2027
- the eventual sale confirmation and parcel identification
Do not simply apply CPI to the number shown on a brokerage dashboard. The legislation limits which cost-base elements are indexed, and managed-fund adjustments can move the tax cost base independently. Use the fund's AMMA statement and the ATO's implementation guidance, or a registered tax agent, for the final calculation.⁴ ⁵
Why ETF Distributions Can Reduce Your Cost Base#
Not all ETF distributions are straightforward income. Some distributions contain components that are not immediately taxable. Two common examples are:
Return of capital: the fund distributes some of the investors' own capital rather than income or gains. This is not assessable income (you are receiving your own money back). However, it reduces your cost base because the invested amount has effectively decreased.
Tax-deferred amounts: certain distributions from managed trusts are structured so that the tax liability is deferred to the future. These amounts are not included in your income in the year received, but they reduce the cost base, bringing forward the tax liability to the point of eventual sale.
Both types of distribution are flagged in the AMMA statement with specific labels, typically "tax-deferred amount" or "cost base reduction amount."
How the Adjustment Works#
The cost base adjustment is calculated per unit and then applied to your holdings.
Example:
You hold 500 units with a cost base of $20.00 per unit (total cost base: $10,000).
The AMMA statement shows a cost base reduction of $0.15 per unit.
Updated cost base per unit: $20.00 − $0.15 = $19.85.
Updated total cost base: 500 × $19.85 = $9,925.
When you eventually sell your 500 units at, say, $25.00 per unit ($12,500 total):
Capital gain = $12,500 − $9,925 = $2,575.
Without the adjustment applied, the incorrect calculation would be:
Capital gain = $12,500 − $10,000 = $2,500.
The difference is $75 in additional capital gain because the cost base was not properly reduced. Over years of holding with multiple adjustments, the cumulative error grows.
Upward Cost Base Adjustments#
Most common are reductions (downward adjustments). However, upward adjustments can occur under the AMIT regime in certain circumstances, such as when the fund's taxable income for the year exceeds the cash it distributed (the excess is attributed to investors for tax purposes, with a matching cost base increase).
An upward cost base adjustment means your recorded cost base increases. This reduces the future capital gain when you sell. The tax on the income was paid in the year of attribution, so the cost base increase prevents that income being taxed again as a capital gain.
Tracking Adjustments Across Multiple Purchases#
The cost base adjustment needs to be tracked for each parcel of units separately. If you have purchased units at different times and prices, each parcel has its own cost base.
Example:
| Parcel | Units | Cost Base Per Unit | Date Acquired |
|---|---|---|---|
| Parcel 1 | 200 | $18.00 | Jan 2022 |
| Parcel 2 | 300 | $21.00 | Aug 2023 |
If the AMMA statement shows a cost base reduction of $0.20 per unit, both parcels are adjusted:
| Parcel | Adjusted Cost Base Per Unit |
|---|---|
| Parcel 1 | $18.00 − $0.20 = $17.80 |
| Parcel 2 | $21.00 − $0.20 = $20.80 |
If you later sell only Parcel 1 (200 units), you use $17.80 per unit as the cost base for that parcel, not $21.00 and not an average.
This per-parcel tracking is also why DRP acquisitions (from distribution reinvestment plans) create additional parcels, each with their own cost base and acquisition date.
A Practical Record-Keeping System#
The minimum requirement is a spreadsheet with one row per parcel, updated annually:
| Column | Contents |
|---|---|
| ETF Name | Fund name or ASX code |
| Purchase Date | Date units were acquired |
| Units | Number of units in this parcel |
| Cost Base Per Unit | Starting cost, adjusted each year |
| Total Cost Base | Units × Cost base per unit |
| Notes | Source (purchase, DRP, etc.) |
After each AMMA statement:
- Identify the cost base adjustment amount per unit (upward or downward).
- Apply it to each parcel for that ETF.
- Update the cost base per unit column.
- Note the financial year and the adjustment source.
What Happens When the Cost Base Hits Zero#
If you hold an ETF for many years and receive many tax-deferred distributions, the cumulative reductions could, in theory, reduce a parcel's cost base to zero.
If additional reductions are reported after the cost base has reached zero, the ATO's general position is that those further reductions become immediately assessable as a capital gain. The tax that was being deferred can no longer be deferred because there is no remaining cost base to absorb it.
This is most likely to occur in funds with a high proportion of tax-deferred distributions (some property and infrastructure funds have historically had this characteristic). It is uncommon for standard equity index ETFs, where most distributions are income (dividends) rather than return of capital.
Tax Deferred Distributions and DRP#
If you participate in a DRP (distribution reinvestment plan) and part of the distribution is tax-deferred or a return of capital, the DRP creates a new parcel of units with its own cost base. The original parcels held prior to the distribution still receive the cost base reduction.
The new DRP parcel's cost base is the market value at which the DRP units were issued, regardless of any tax-deferred classification of the distribution. This is an area where record keeping becomes particularly layered, and maintaining accurate notes from each distribution event is worth the effort.
Summary#
ETF distributions can create upward or downward cost-base adjustments that are reported on annual AMMA statements. Failing to record them can misstate the capital gain or loss when you sell. From 1 July 2027, eligible investors also need records supporting the parcel's pre-reform component, transition value and indexed post-reform component. The new indexation rules do not replace AMMA adjustments. Maintain one continuous parcel-level history from purchase through the transition and eventual sale.