The Coalition's proposed Tax Back Guarantee would automatically index personal income tax thresholds to inflation, changing the way bracket creep operates in Australia's tax system.

The proposal begins with the two lower income-tax thresholds from 2028–29 before extending indexation across all four thresholds from 2031–32. The Coalition has stated that the initial stage would cost about $22.5 billion over its first four years and that offsetting measures would be identified before the 2028 election. The proposal remains opposition policy rather than enacted law.

The longer-term budget effect is potentially much larger because threshold indexation is cumulative. Each annual increase permanently lifts the starting point for future tax calculations.

What the Tax Back Guarantee Would Do

Australia's personal income tax thresholds are currently fixed in nominal dollar terms unless Parliament changes them.

As wages rise, taxpayers can therefore pay a larger proportion of their income in tax even when part of the wage increase merely reflects inflation.

This effect is commonly known as bracket creep.

Under the proposed Tax Back Guarantee, thresholds would instead rise automatically with inflation.

The bottom two thresholds would begin being indexed from 2028–29.

From 2031–32, the proposal would extend indexation to all four personal income-tax thresholds.

The policy is intended to limit increases in average tax rates caused solely by inflation.

Why Bracket Creep Raises Revenue

Bracket creep occurs because nominal wages tend to increase over time while tax thresholds remain fixed.

Consider a simplified worker whose income rises from $44,000 to $46,000.

Part of the additional income can move into a higher marginal tax bracket even where the worker's purchasing power has changed relatively little.

Across millions of taxpayers, this process progressively increases personal income-tax receipts.

The Parliamentary Budget Office projects that, under existing policy settings, the average personal income-tax rate will rise from 24.9% in 2026–27 to 28.6% by 2036–37, with bracket creep accounting for most of the increase. Personal income tax is also projected to rise from 47.9% to 53.8% of total Commonwealth revenue over the same period.

Why Indexation Has a Growing Budget Cost

Indexing tax thresholds does not operate like a one-off tax cut.

Suppose a threshold increases by 2.5% in one year.

The following year's indexation is then applied to that already higher threshold.

The process repeats each year.

Over time, the difference between indexed thresholds and thresholds that remain fixed becomes increasingly large.

That means the revenue forgone also tends to increase as the policy matures.

The effect becomes larger again once the proposed indexation extends to the higher tax thresholds from 2031–32.

PBO analysis of CPI indexation scenarios confirms that returning bracket creep would materially reduce projected Commonwealth revenue and weaken the budget balance unless other revenue increases or spending reductions offset the effect.

Why the Four-Year Figure Is Only Part of the Picture

The Coalition's stated $22.5 billion figure relates to the initial implementation period.

That period captures only the early phase of the policy.

It does not represent the permanent annual cost once indexation has operated for several years and all four thresholds have entered the system.

This distinction is important when assessing any permanent tax policy.

A four-year budget estimate answers one question: how much revenue is forgone during the initial forward-estimates period.

A longer-term projection answers another: how large the annual effect could become once the policy is fully established.

Both can be useful, but they measure different periods.

PBO Modelling Shows the Longer-Term Effect

The Parliamentary Budget Office separately models what happens to the Commonwealth budget when personal income-tax thresholds are indexed to CPI.

Its medium-term analysis shows that CPI indexation would materially weaken the projected budget position compared with maintaining fixed thresholds.

The PBO's modelling is broader than the Coalition's staged proposal, so it should not be treated as a formal costing of the Tax Back Guarantee itself.

The distinction matters because a formal costing would need to incorporate the exact commencement dates, threshold settings, inflation assumptions, existing legislated tax cuts and any proposed offsets.

The PBO's published scenario analysis nevertheless demonstrates why the fiscal effect of permanent indexation grows over time.

Inflation Is a Key Variable

The eventual cost of indexation would depend heavily on inflation.

If consumer prices increase rapidly, tax thresholds would rise faster.

That would leave more income taxed at lower marginal rates compared with a system where thresholds remained fixed.

The resulting reduction in tax receipts would therefore be larger.

If inflation were lower, the annual threshold increases and associated revenue effect would also be smaller.

This means long-range estimates are sensitive to economic assumptions.

A Hypothetical Example of the Taxpayer Effect

A simplified example shows how threshold indexation can produce an annual tax saving.

Assume the tax-free threshold of $18,200 and the next threshold of $45,000 are both indexed upward by 2.5%.

The first threshold would rise to approximately $18,655.

The second would increase to approximately $46,125.

For a taxpayer earning more than $46,125, an additional $455 would fall below the tax-free threshold.

At a 14% marginal rate, that represents approximately $64 of tax difference.

A further $1,125 would remain in the lower 14% bracket rather than moving into a 30% bracket.

That produces a difference of approximately $180.

Together, the simplified annual benefit would be around $244.

Actual outcomes would depend on the tax rates and thresholds applying at the time, inflation and the taxpayer's income.

Existing Tax Cuts Form the Baseline

Any future indexation policy would operate on top of tax changes that are already legislated.

From 1 July 2026, the marginal rate applying to taxable income between $18,201 and $45,000 fell from 16% to 15%.

It is legislated to fall again to 14% from 1 July 2027.

A Working Australians Tax Offset of up to $250 is also scheduled to apply from the 2027–28 income year.

These settings would form part of the baseline against which any future indexation proposal would need to be costed.

Why Funding Measures Matter

Permanent indexation reduces the amount of revenue that would otherwise arise through bracket creep.

If that lost revenue is not replaced, the effect flows through to the Commonwealth budget balance.

The PBO's medium-term analysis specifically identifies increasing personal income-tax receipts, predominantly through bracket creep, as an important contributor to projected fiscal improvement.

The Coalition has stated that the initial cost of its proposal would be offset by other measures.

The fiscal impact therefore cannot be fully assessed until those measures are specified and incorporated into a complete costing.

The Proposal Is Not Yet Law

The Tax Back Guarantee is an opposition policy proposal.

Automatic indexation of personal income-tax thresholds has not been enacted.

Under current arrangements, Parliament retains the ability to change tax rates and thresholds through legislation.

If the proposed policy were ultimately implemented, the final fiscal effect would depend on the legislation, commencement dates, inflation and any associated spending or revenue measures.

What to Watch

One key development will be whether a formal policy costing is released using the final design of the Tax Back Guarantee.

That would allow the staged proposal to be distinguished more clearly from broader modelling scenarios involving full CPI indexation.

The treatment of offsetting measures will also matter because their size and timing would determine the net effect on the Commonwealth budget.

Future inflation forecasts are another important variable because they directly affect how quickly indexed thresholds would rise.

For now, the Coalition's four-year figure describes the initial phase of its proposal, while PBO analysis illustrates why permanent threshold indexation has a substantially larger fiscal effect over the medium term.