How is child support calculated in Australia?

Child support in Australia is calculated by Services Australia using a formula set out in the Child Support (Assessment) Act 1989 (Cth), which combines both parents' incomes with the amount of care each parent provides. It is an administrative calculation, not a judgment about who was the better parent: the two things that move the number are income and nights of care. Separated parents across Melbourne and Victoria ask us this more than any other question — usually followed by a second: what can I do if the figure is wrong?

How does Services Australia work out child support?

The formula runs in a set sequence. For the most common situation — two parents, one child support case — it works like this:

1. Each parent's child support income is their adjusted taxable income, less a self-support amount (a set figure updated annually to cover their own living costs) and less allowances for other dependent children they support.
2. The two child support incomes are added together to give a combined child support income.
3. Each parent's share of that combined figure is their income percentage.
4. Each parent's percentage of care is worked out, usually by counting nights per year.
5. Care percentage converts to a cost percentage — the share of the children's costs each parent is treated as already meeting by having the children with them.
6. Cost percentage is subtracted from income percentage. A parent with a positive result is the paying parent.
7. The costs of the children are read from an annual table based on the parents' combined child support income and the number and ages of the children.
8. The paying parent's percentage is applied to those costs to give the annual rate of child support.

Two consequences explain most of the surprises. The calculation runs on taxable income, so a parent whose real position is better than their tax return suggests can produce a figure that feels wrong; and because care is netted off against income, a change in nights moves the assessment even when no income has changed. Variations of the formula apply where a non-parent cares for a child, where a parent supports children in more than one case, or where a parent lives overseas.

How does the amount of care change child support?

Care is measured as a percentage of the child's time, generally counted in nights across the year, and it directly reduces what a parent is assessed to pay. Services Australia works in bands: regular care is 14% up to less than 35% of care, and shared care is 35% to 65% — 35% being roughly 128 nights a year.

The bands create thresholds. A parent with less than 14% of care is generally not credited with any cost percentage, so occasional contact does not reduce what they pay, and crossing into shared care changes the calculation meaningfully. That is why arguments about nights become arguments about money, and why arrangements are worth recording properly — our guide to parenting arrangements after separation covers how.

One point has to be said plainly: care and money are separate legal obligations. Withholding the children is not a lawful answer to unpaid child support, and refusing to pay is not a lawful answer to withheld time.

What if the assessment does not reflect reality?

Where the formula produces an unfair result because of special circumstances, a parent can apply to Services Australia for a change of assessment. There are 10 prescribed reasons; an application must fit one of them and be supported by evidence — being unhappy with the amount is not a reason.

The reasons cover what the formula cannot see: the high costs of spending time with or communicating with a child, a child's special needs, a duty to maintain another child or person, and — the one most often argued — that a parent's income, property or financial resources do not reflect their real capacity to support. That last category is where earning capacity, income from a family trust or company, or assets producing no taxable income come into account. These decisions are discretionary and evidence-driven, and both parents get to respond.

How do I object to a child support decision?

The first step is an objection to Services Australia, and you generally have 28 days from receiving the decision to lodge it — 90 days if you live in a reciprocating jurisdiction overseas. Different arrangements can apply to care percentage decisions, so read the decision letter, which sets out your review rights.

If that does not resolve it, the next step is the Administrative Review Tribunal, the independent Commonwealth tribunal that replaced the former Administrative Appeals Tribunal. You generally have 28 days from the objection decision to apply, and the Tribunal can extend that if you apply in writing and explain the delay. The most common reason a parent loses a good argument is letting these short windows run out.

Can parents agree on child support instead?

Yes, and for many separated parents an agreement beats living with an annual recalculation. There are two forms:

• Binding child support agreements can be for any amount — including less than the formula figure — and can cover non-cash items such as school fees or health insurance. The trade-off is formality: each parent must receive independent legal advice from an Australian legal practitioner before signing, and a binding agreement is hard to get out of later, which is the point of it.
• Limited child support agreements need no legal advice, but an administrative assessment must already be in place and the agreed amount must be at least what that assessment produces.

They fail for the same reasons binding financial agreements do: rushed drafting, advice that was not genuinely independent, or terms that do not survive a change nobody planned for.

When does child support stop?

A child support assessment normally ends when the child turns 18. If the child is still in full-time secondary study, a parent can apply to extend it to the end of that school year — but the application has to be made while the child is 17, a deadline that quietly catches parents out every year.

After that, Services Australia is no longer the avenue: support for an adult child requires a court order from the Federal Circuit and Family Court of Australia, available where maintenance is necessary to enable the child to complete their education or because of the child's physical or mental disability. As a rule, apply before the child turns 18.

What happens if a parent does not pay?

Enforcement depends on how payments are collected. Parents can transfer payments between themselves (private collection) or ask Services Australia to collect. Where Services Australia collects and a parent falls behind, it has real powers: deducting child support from wages through the employer, intercepting tax refunds, issuing departure prohibition orders that stop a parent leaving Australia, and legal action where other methods fail. If you are collecting privately and payments have stopped, switching to Services Australia collection is usually the first practical move — those powers only become available once the agency is collecting.

What is changing in the child support system?

As at August 2026, the Australian Government has announced a package of reforms to the Child Support Scheme, described as the most significant changes since 2008 and to be implemented progressively from 2026. The announced measures target non-compliance and the use of child support as a tool of financial abuse: greater use of employer withholding, faster disbursement of collected payments to the receiving parent, stronger enforcement against large debts including international travel restrictions, and an end to the mandatory open exchange of information in change of assessment and objection processes.

Most of these depend on legislation and commence on staged dates, so treat them as announced rather than as the law today. Everything above reflects the system as it operates as at August 2026.

Practical steps if you are starting out

1. Apply promptly. A new assessment generally starts from the day Services Australia receives the application, not from the date you separated.
2. Keep a care diary. Nights are the second lever in the formula, and contemporaneous records beat recollection.
3. Keep tax lodgements current. If a parent has not lodged, the assessment may run on estimated or default income figures.
4. Diarise the deadlines — the 28-day objection window, and the extension application while a child is 17.

How we help

Spencer Alexander Lawyers advises separated parents across Melbourne and Victoria on child support: whether a change of assessment is genuinely arguable on your facts, how to prepare an objection or Administrative Review Tribunal application within time, and whether an agreement gives you the certainty you are actually after. We also act for the parent responding to an application. Child support rarely travels alone, so we deal with it alongside property settlement and parenting arrangements. Learn more about our family law practice.

Common questions

How is child support calculated in Australia? Services Australia applies a formula under the Child Support (Assessment) Act 1989 (Cth). It deducts a self-support amount from each parent's adjusted taxable income, combines what is left, works out each parent's share of that combined income, and converts each parent's percentage of care into a share of the children's costs. The parent whose income share exceeds their share of those costs pays the difference.

Can parents agree on a different amount of child support? Yes. A binding child support agreement can be for any amount, including less than the formula figure, but each parent must first receive independent legal advice from an Australian legal practitioner. A limited child support agreement does not require legal advice, but an administrative assessment must already be in place and the agreed amount must be at least what that assessment produces.

Is child support taxable income in Australia? No. Child support is not assessable income for the parent who receives it and is not tax deductible for the parent who pays it. It can still affect Family Tax Benefit Part A, because the child support a parent is entitled to receive is taken into account in working out that payment.

When does child support stop in Australia? A child support assessment normally ends when the child turns 18. If the child is in full-time secondary study, a parent can apply to extend it to the end of that school year, and that application has to be made while the child is still 17. Support for an adult child instead requires a court order from the Federal Circuit and Family Court of Australia, available where maintenance is necessary to enable the child to complete their education or because of the child's physical or mental disability.

This guide reflects the law applying in Victoria as at August 2026. It is general information only, not legal advice, and does not take your circumstances into account.