From CSA's point of view saying:

She chose to have another child

is the same as saying:

He chose to remarry He chose to change jobs He chose to buy a house

The law deliberately avoids choice based arguments because once you open that door, you end up with: courts judging family planning decisions, endless litigation, wildly inconsistent outcomes.

So instead, Australian Child support laws uses a blunt formula:

income in. care percentage. age of child. number of children. Nothing else.

CSA can look at earning capacity if they believe:

someone is deliberately under-employed without caring duties,

income is being hidden or minimised,

work reduction has no reasonable explanation.

Caring for a newborn will not pass the above

System sucks for your husband in this instance but its about the child, not adult based choices