How is child support calculated in Australia?

Services Australia runs an 8-step formula. Each parent's Child Support Income equals their adjusted taxable income minus the Self-Support Amount ($31,046 for 2026). Both parents' CSIs combine, each gets an income percentage and a care percentage, the care % maps to a cost percentage, and the formula calculates the transfer based on the higher-earning / lower-caring parent's "share gap". The result is then checked against the Minimum Annual Rate floor + Fixed Annual Rate floor.

What's the minimum child support payment?

Two floors apply. The Minimum Annual Rate (MAR) for 2026 is $519 per case — the absolute minimum payable when the formula says less. The Fixed Annual Rate (FAR) is $1,720 per child (capped at 3 children, so $5,160 max) — applies when the paying parent has zero income support, ATI below the Pension PP Single ($25,225), and less than 35% care. FAR is checked FIRST and overrides the formula result when it applies.

Does my new partner's income affect my child support?

No. The child support formula uses only the two biological/adoptive parents' incomes. New partners, de facto spouses, step-parents — none of them enter the formula. (Centrelink family payments work differently — your new partner's income DOES count for FTB and CCS purposes.) See our step-parent income guide for the full detail.

What is the Self-Support Amount?

It's the floor each parent gets to keep before any child support is calculated. For 2026 the SSA is $31,046. Your ATI minus SSA equals your Child Support Income — the figure the formula actually uses. Indexed each year to inflation. Same SSA applies to both paying and receiving parents.

What's the Cost of Children table?

A government-published lookup table that estimates the annual cost of raising children at each income band. The 8-step formula uses it to calculate the total cost of the children at the combined parental income, then divides that cost between parents based on their respective income and care percentages. Higher combined income means higher assumed cost per child.

Can I change my child support assessment?

Yes, via the Change of Assessment process — 10 specified grounds (high contact costs, special needs, education costs, child has own income, parent has financial resources beyond declared income, earning capacity differs from declared, etc.). It's free, decisions usually within 90 days, and you can appeal to the Administrative Appeals Tribunal if dissatisfied. See our Change of Assessment guide for the full process.

How is child support worked out in Australia? The 8-step formula in plain English

Services Australia's child support assessment is an 8-step formula based on both parents' income, care percentages, and the Cost of Children table. Here's exactly how it works in 2026, with a worked example.

Australian child support is worked out by Services Australia's Child Support Agency using an 8-step formula in the Child Support (Assessment) Act 1989. The headline drivers are both parents' adjusted taxable incomes, the percentage of nights each child spends with each parent, and a Cost of Children table that translates combined parental income into a dollar cost per child. From that, each parent's "share" is calculated based on their income share minus their care share. For 2026, the Self-Support Amount each parent gets to keep before any child support is calculated is $31,046 (⅓ × the annualised Male Total Average Weekly Earnings of $93,137). This guide walks through all 8 steps with a worked example.

Who this applies to

Child support applies when:

You and the other parent are separated (or were never together).
You have one or more children together under 18 (or up to 19 if still in secondary school).
One of you has applied to Services Australia for a child support assessment (most do, even when the eventual arrangement is private).

If you have a private agreement outside the Child Support Agency, the formula here still gives you the figure the agency would assess at — useful as a sanity check against what you've agreed.

Try the free CS calculator → · Open the full estimator → Enter both parents' incomes and care percentages and the estimator gives you the exact assessment figure for 2026 — same formula Services Australia uses.

The 8 steps

Step 1 — Each parent's Child Support Income. For each parent: take their Adjusted Taxable Income (ATI) (see our ATI guide), subtract the Self-Support Amount of $31,046 (the floor the parent gets to keep), and subtract any relevant dependent child allowance for kids they're already supporting in their current household. Result: their Child Support Income (CSI).

A parent whose CSI works out negative has CSI = $0 for the formula.

Step 2 — Combined Child Support Income. Add both parents' CSI together. This is the family-pool income that drives the cost-of-children calculation.

Step 3 — Income share for each parent. For each parent: their CSI ÷ combined CSI = their percentage share of income. E.g., if Parent A has $40,000 CSI and Parent B has $20,000, A's income share is 66.67% and B's is 33.33%.

Step 4 — Care percentage for each child. Based on the actual or expected pattern of overnight stays for each child:

≥ 65% of nights = primary carer.
35–65% = shared care.
< 35% = regular carer (also a "below-regular-care" zone below 14%).

Care percentage matters because it determines who is paid and how much.

Step 5 — Cost percentage for each parent. Care % gets converted into a cost percentage using a 7-band table from the Child Support Guide §1.1.C.200. Roughly:

Care %	Cost %
< 14%	0%
14–34%	24%
35–47%	25% + (care% − 35) × 2
48–52%	50%
53–65%	51% + (care% − 53) × 2
66–86%	76%
> 86%	100%

Step 6 — Cost of the children. Look up the Cost of Children from the Government's 2026 table using combined CSI and the number/age of children. For example, a couple with combined CSI of $90,000 and one child aged 0–12 has a Cost of Children around $13,000/year (the exact figure depends on the band; the calculator gives the precise number).

Step 7 — Each parent's share of the cost. Each parent's share = income % − cost %. If the result is positive, that parent pays child support. If negative, they receive it.

Step 8 — Floor checks: MAR and FAR.

The Minimum Annual Rate (MAR) is $551 for 2026 ($10.60/wk). Most assessments above-zero income are at least this amount.
The Fixed Annual Rate (FAR) of $1,825 for 2026 applies in certain low-income / Centrelink-payment scenarios.
A parent with very low income may have their assessment set to MAR or FAR rather than the formula result.

A worked example

Parent A earns ATI $80,000. Parent B earns ATI $45,000. They have one child aged 6, who lives with A 75% of the time and B 25% of the time. They have no other dependent children in either household.

Step 1 — CSI:

A: $80,000 − $31,046 = $48,954
B: $45,000 − $31,046 = $13,954

Step 2 — Combined CSI: $48,954 + $13,954 = $62,908

Step 3 — Income shares:

A: $48,954 / $62,908 = 77.82%
B: $13,954 / $62,908 = 22.18%

Step 4 — Care %:

A: 75%; B: 25%

Step 5 — Cost %:

A: 75% care = band "66–86%" = 76% cost share.
B: 25% care = band "14–34%" = 24% cost share.

Step 6 — Cost of Children: for combined CSI $62,908 and one child aged 6, the COTC formula gives approximately $11,300/year (the exact figure depends on the precise band; our estimator computes it from CSG §2.3.1.20).

Step 7 — Each parent's share of cost:

A's share: 77.82% income − 76% cost = +1.82% → A pays support of 1.82% × $11,300 ≈ $206/yr (about $4/wk).
B's share: 22.18% income − 24% cost = −1.82% → B receives support of about $206/yr.

Step 8 — Floor: $206/yr is below the MAR of $551/yr. The assessment would round up to $551/yr ($10.60/wk).

This illustrates why most shared-care arrangements where care % roughly matches income % produce small or minimum-rate assessments. The formula doesn't ask one parent to fund the other's lifestyle — it asks each to contribute proportionally to the cost.

The four gotchas that catch families out

1. Care % is OVERNIGHT-based, not "time spent." Centrelink works in nights, not days or hours. A weekend (Friday night + Saturday night) is 2 nights = ~7.7% over a fortnight, not "weekend = half a week."

2. The Self-Support Amount changes every January. The 2026 figure ($31,046) is current from 1 January 2026. The 2025 figure was $29,841; the 2024 figure was $28,463 — it indexes with MTAWE annually.

3. The Cost of Children depends on combined income, not yours alone. Two families with the same children but very different combined incomes have very different Cost of Children — Centrelink assumes higher-income families spend more on children.

4. Receiving child support reduces your FTB Part A. Above a threshold, FTB-A is reduced by 50¢ for every $1 of child support received — see the Maintenance Action Test for the full picture.

How NestWise calculates child support

NestWise's Child Support Estimator implements all 8 steps using the exact figures above:

CSI per parent (ATI − SSA − relevant dependent allowance).
Combined CSI and income shares.
Care % per child (overnights/year basis).
Cost % via the 7-band table.
Cost of Children from the 2026 COTC table (CSG §2.3.1.20).
Pay/receive amounts per parent.
MAR / FAR floor checks applied.

The full source list is on the sources page, including the Government's authoritative Child Support Guide. The estimator handles multi-child families and Formula 3 (multi-case) scenarios where a parent supports children in more than one household.