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Family Law · by Peita Savage · 5 October 2026

Child support: how it's worked out

Child support is calculated by a formula, not by argument. Here's what goes into it and what to do when the formula doesn't fit your situation.

Child support is one of the most misunderstood parts of separation. People expect a battle; in fact, the starting point is a formula run by Services Australia, based on a small number of facts about both parents.

What the formula uses

The assessment combines both parents' incomes, the number of children and their ages, and the percentage of nights the children spend with each parent (the 'care percentage'). Each parent gets a self-support amount set aside first; the rest is shared in proportion to income and care.

Why care nights matter so much

Because care percentage directly drives the amount, small changes in the parenting arrangement can change child support significantly. This is one reason parenting and child support are best looked at together, not in isolation.

When the formula doesn't fit

The standard formula assumes fairly typical circumstances. If a child has special needs, high medical or education costs, or a parent's true income is not reflected in their tax return, you can apply for a 'change of assessment' on one of several recognised grounds.

Private agreements

Parents can also make their own arrangements through a binding or limited child support agreement, which can include things the formula doesn't — like school fees or health insurance paid directly. A binding agreement requires legal advice for each parent.

Where we help

We do not run the Services Australia calculator for you — that's free and online. Where we add value is the harder cases: change-of-assessment applications, binding agreements, and situations where child support, parenting and property all interact. If your situation is not 'standard', it is worth a conversation.

This article is general information only and is not legal advice. For advice about your situation, please contact HT Law Services.

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General information only, not legal advice. For advice on your circumstances, contact HT Law Services on (02) 9280 1548.