Child support, explained

My first child support assessment landed in my inbox at 6.14am on a Tuesday. The number wasn't what I'd budgeted for. I made coffee, opened the calculator, and realised I'd been working off a back-of-the-envelope figure my mate at work had given me. His situation was nothing like mine. The actual formula is public, predictable, and worth understanding before you build your post-separation budget.

Child support in Australia is administered by Services Australia (the agency formerly known as the Child Support Agency, still called the CSA by everyone over 35). It's a federal scheme, formula-driven, and largely takes negotiation off the table. That's a feature, not a bug.

Why a formula exists

Before 1989, child maintenance was set case-by-case by family courts. It was slow, inconsistent, and chronically under-enforced. The Child Support (Assessment) Act 1989 introduced an administrative formula that produces a number from inputs both parents already report to the ATO.

The formula is opaque if you've never seen it explained, but it's deterministic. Same inputs, same answer, every time. There's an online estimator on the Services Australia website. Use it.

The eight-step formula

Services Australia applies the following steps to calculate the annual child support amount. You don't need to do the maths yourself. You do need to understand what moves the number.

1. Each parent's adjusted taxable income is calculated (taxable income plus reportable fringe benefits, super, investment losses, foreign income).
2. A self-support amount is subtracted from each parent's income (around $30,000 in 2026, indexed annually).
3. The remaining amounts are added together to get combined child support income.
4. Each parent's percentage share of that combined income is calculated.
5. The percentage of nights each parent has the child is calculated, then converted to a 'cost percentage' using a non-linear lookup table (more nights = disproportionately more cost).
6. The cost of the child is calculated based on combined income and the number/age of children, using published cost tables.
7. Each parent's share of the cost is calculated by subtracting their care percentage from their income percentage.
8. Whichever parent has the positive number pays. The other parent receives.

If both parents earn similar amounts and share care close to 50/50, the payment can be small or even zero. If incomes are unequal or care is unequal, the payment grows.

What moves the number

Three inputs matter most:

• Your taxable income (lower means lower payments, but the ATO is the ATO)
• Your ex's taxable income (higher means lower payments from you)
• The number of nights you have the children per year

Care percentage thresholds matter enormously. The lookup table groups care into bands:

• Below 14% (less than 52 nights/year): 0% cost percentage
• 14% to 34% (52-127 nights): 24% cost percentage
• 35% to 47% (128-175 nights): 25% to 49% sliding
• 48% to 52% (176-189 nights): 50% cost percentage
• 53% to 65%: 51% to 75% sliding
• 66%+: 76% cost percentage

The big jumps are at 14% (where you start being recognised as having care at all) and at 35% (where 'shared care' kicks in and significantly reduces the payment from the higher-earner). Below 14%, the formula treats you as a non-resident parent paying full freight.

Tax-return inputs

The system pulls your income directly from your most recent tax return. If you're self-employed and your income varies, you can:

• Estimate your income for the current year (binding once accepted, with reconciliation against actuals later)
• Apply for a change of assessment if your circumstances have materially changed
• Provide an interim income figure if you've lost a job or had a major income drop

Failing to lodge tax returns triggers a default income figure, often higher than your actual income. Stay current with the ATO.

The 'capacity to earn' rule

If Services Australia or the other parent believes you've deliberately reduced your income to lower payments (resigning to take a low-paid job, hiding income through a business), they can apply a 'capacity to earn' assessment. The agency assigns an income figure based on what you could be earning, not what you are. This is a common (and often appropriate) tool used against parents who try to game the system.

The reverse also applies. If your ex has artificially reduced her income, you can request a change of assessment on the same grounds.

Change of assessment (Reason 8 and friends)

The formula doesn't fit every situation. The Act recognises 10 'reasons' for departure from the formula assessment. The most commonly used:

• Reason 1: Costs of spending time with the child (high travel, e.g. interstate access)
• Reason 2: Special needs of the child (disability, medical, gifted education)
• Reason 3: Other necessary expenses for the child (private school fees, where one parent insists)
• Reason 8: A parent's income, property, or financial resources don't accurately reflect their financial position (capacity to earn cases)

You apply via Services Australia, both parties make submissions, a Senior Case Officer decides, and the assessment is varied if the application succeeds. Decisions can be reviewed by the Administrative Review Tribunal.

Private collect vs Services Australia collect

Once an assessment exists, you can choose how the money moves:

• Private collect: you pay your ex directly (bank transfer, BPAY). Services Australia tracks the assessment but doesn't handle the cash. Lower friction. Suits cooperative ex-couples.
• Agency collect: Services Australia takes the payment from your wages or bank account and pays it across. Use this if there's a history of late or missed payments, or if your ex is on Centrelink (which often requires it).

You can switch from private to agency collect if payments fall behind. The agency has serious enforcement tools: garnishee orders, tax refund interception, departure prohibition orders (preventing overseas travel). They use them.

Periodic vs lump-sum

The default is periodic payments (monthly). You can also pay child support as a lump sum, credited against the assessment over time. This sometimes makes sense in property settlement contexts where you'd rather front-load the payment from the asset pool than carry a recurring liability for years.

A binding child support agreement signed with independent legal advice can also vary the formula amount, including providing for non-cash benefits like school fees or health insurance. Get advice before signing one. They're hard to undo.

What child support doesn't cover

Child support is intended to cover the day-to-day costs of raising the child: food, clothing, housing share, transport, basic activities. It is not designed to cover:

• Private school fees (typically negotiated separately, often via consent orders)
• Major medical expenses not covered by Medicare or private health
• Extra-curricular activities at the higher end (elite sport, music, tutoring)
• Overseas school trips

These are typically handled by separate written agreement or as orders within the property settlement.

What to actually do

• Run the Services Australia estimator with realistic inputs before you finalise anything
• Keep your tax returns lodged and up to date
• Document care nights accurately from day one
• Don't try to game the system; Reason 8 exists and the agency is good at finding shoes
• Reassess annually when your tax return updates or care patterns change

The formula is fairer than you think and harsher than you'd hope, depending on which side of it you're sitting.

Run the numbers. Pay on time. Move on.

Further watching

• 4 Child Support Recommendations (Family Law Inquiry)
• To Lawyer Up or Not When Dealing with the Child Support Office