The simplest answer to one of the most-asked questions in Australian family finance: NO, a step-parent's (or new partner's) income does not affect child support. The formula uses only the two biological/adoptive parents' adjusted taxable incomes. Marriage, de facto status, cohabitation — none of it enters the calculation. This is the case both for the paying parent (your new partner doesn't help you pay) and the receiving parent (their new partner doesn't reduce what they get).
The one nuance worth knowing: while step-parent income is invisible to the FORMULA, it can be considered in a Change of Assessment review under specific grounds. And separately, your Family Tax Benefit does change when you re-partner — Centrelink counts your new partner's income fully for FTB purposes. So your overall family-payment picture shifts after re-partnering even though your CS doesn't.
This guide explains exactly where the formula draws the line, why it draws it there, and the (narrow) situations where partner income IS considered. Verified against Services Australia and the Child Support Guide §2.4.
What the formula actually uses
The 8-step child support formula uses only these inputs:
- Parent 1's ATI (from their last lodged tax return)
- Parent 2's ATI
- Number of children in the case
- Each child's age (0–12 vs 13–17 vs 18+ in study)
- Each parent's care percentage for each child
- The relevant year's Self-Support Amount ($31,046 for 2026)
- The relevant year's Cost of Children table
Notice what's NOT in the list: partner income, household income, step-children, other dependents (mostly), assets (mostly), housing costs, debts, lifestyle expenses. The formula is narrow on purpose — it's designed to produce a defensible transfer between parents using only their respective abilities to earn.
Why this is the legal position
Under Australian family law, financial responsibility for a child sits with the child's biological or adoptive parents — not with anyone they later marry or partner with. A step-parent can have an emotional and practical role in a step-child's life but doesn't acquire a legal duty to financially maintain the step-child unless they formally adopt.
The relevant statute (Child Support (Assessment) Act 1989) defines "parent" specifically. Step-parents, grandparents, foster parents — none are covered by the CS formula unless they're the legal parent of the child.
Where step-parent income CAN matter: Change of Assessment
The 10 grounds for a Change of Assessment (Child Support Guide §2.6) include two that can touch on partner finances:
- Reason 7 — "The assessment is unjust because of the parent's actual income, earning capacity, property or financial resources." Services Australia can consider whether you have substantial financial resources THROUGH a partner (e.g. living in a $3M home owned by your partner while declaring low income), even though the partner's income itself doesn't get added to the formula.
- Reason 8 — Similar grounds where the parent's actual earning capacity differs from their declared income.
These reviews are case-by-case and the bar is high. Services Australia won't reassess just because your ex re-partnered with a high earner — they'll only act if there's evidence the assessment misrepresents the parent's actual financial situation in a meaningful way.
Try the free CS calculator → · Open the full estimator → See the formula working with just the two parents' incomes — exactly as Services Australia would calculate it.
Where step-parent income DOES count: Centrelink family payments
Once you re-partner (married, de facto, or living together as a couple), Centrelink treats you as a couple for ALL family payments:
| Payment | New partner income counted? |
|---|---|
| Child support transfer | NO (formula uses only the two legal parents) |
| FTB Part A | YES (combined family income test) |
| FTB Part B | YES (primary + secondary earner test; new partner is one of these) |
| CCS | YES (combined family income drives rate) |
| Parenting Payment | YES (couple income test) |
| Rent Assistance | YES (paid with FTB-A; combined family test) |
So the household income picture shifts the moment you re-partner, even though child support doesn't budge. This is why families re-partnering need to update their income estimate with Centrelink immediately — the new partner's income flowing in can push family ATI over a CCS taper threshold or the FTB-A cliff, triggering a debt at EOFY if not declared.
Common scenarios
Paying parent re-partners with someone earning $200k: Your CS assessment doesn't change. The formula still uses just your ATI vs the other parent's ATI. Your FTB-A and CCS for ANY children you and your new partner have together are tested on combined income, but the CS you pay for your kids from a prior relationship stays exactly the same.
Receiving parent re-partners: You still receive the same CS. But your FTB-A and CCS for the kids will be income-tested on the combined household income, so they may reduce. Your CS stays whole; your Centrelink payments shrink as combined income rises.
Both parents re-partner: Both CS assessments unchanged (uses just the two original parents' incomes). Both households' Centrelink payments shift based on the new combined incomes.
Paying parent re-partners and stops working to be a stay-at-home parent for new kids: You can voluntarily reduce work hours, but Services Australia may invoke Reason 8 — your "earning capacity" hasn't changed even if your actual income has, because the work reduction is a personal choice. The CS assessment can be revised upward to reflect what you COULD earn. This is a real and common outcome — voluntary income reduction post-separation is one of the most-litigated grounds.
What about step-children?
If you re-partner and your new partner has children from a prior relationship, those step-children don't enter your CS formula either — neither as dependents you support (which would reduce your CS) nor as dependents the other parent supports. The CS formula only considers the children of the assessment itself.
The exception is the Additional Dependent Children allowance in some formulas, which reduces your CS payment if you have OTHER biological/adoptive children from another relationship that you also support financially. Step-children don't count for this either — only biological / adoptive children.