When a parent has more than one child support case, Services Australia applies Formula 3. Here is how it works and where to start your estimate.
Start with the single-case CS calculator →Formula 1 (the basic Australian child-support formula) applies when both parents have only one child support case with each other. Formula 3 replaces it when a parent has multiple cases — for example, children from two different relationships.
Under Formula 3, Services Australia works out each case separately using the paying parent's income + the Cost of Children table, then allocates a portion of their capacity to each case. The mechanics are the same as Formula 1 for one case at a time — Formula 3 just handles the allocation between cases.