Hold on, don't jump the gun, yet, mate. They weren't calling you to tell you they'd already made a decision. To the contrary, they were just calling to let you know about the application and invite you to provide any documents that you think might help them make the correct decision in the first instance. There's no screening protocol that restricts CSA from considering even the really outlandish claims if they've been made on the right form, so basically, every application has to be considered, but they will always seek input from the other parent before even commencing that consideration process.
Now, the key thing to remember, as well, is that CSA doesn't just do whatever a parent tells it to. Even if you don't provide any evidence, CSA still has very strict rules to follow and very strict legislation to adhere to, and going by what you've said previously I don't think her claim is going to make the cut on either of those fronts.
If they do make a decision that favours her, you can request a review by the Administrative Appeals Tribunal (AAT).
The AAT is not the same as 'fighting it in Court'. In fact, the AAT isn't a Court at all. Requesting a review by the AAT is free, and it will hardly ever allow either party to a dispute to have a lawyer represent them, meaning both parents usually must be self-represented. The process is intended to be an informal review to check that CSA actually followed the Child Support (Assessment) Act when it made its decision. It's not a legal dispute between parents.
Now, I've helped my husband with AAT proceedings for one child support decision, and let me tell you, I found the AAT to be far more objective and considerate of case facts than the CSA had ever been, because the CSA follows its own policies procedures, whereas the AAT actually follows the child support legislation, which is far more flexible and responsive than the CSA pretends it to be.
Short form, don't be afraid of the AAT. It costs nothing, and that's not an exaggeration. You can attend by phone, the hearing takes less than an hour in most cases, and it's just you, your ex and the Tribunal member having a discussion about the facts and how the legislation applies to them. Simple.
But really, it probably won't get that far. I'd be surprised if CSA made a decision in her favour on this one, but you do need to make sure you provide evidence just so any possible discrepancies in the facts are properly brought to light before a decision is made.