I still stick to my initial opinion that Ameer should seek legal advice from a suitably qualified and experienced legal practitioner. To be clear, while I am qualified I am not experienced in this area of law.
If I had to guess, this is one scenario which a legal practitioner may advocate - based solely on the information we know and accepting it all as correct:
1. Report your ex to Department of Human Services and the Australian Taxation Office for undeclared income. Provide whatever evidence you can. If she had an online business, this is a good place to start as there will have had to be a paper trail for the money. Don’t threaten it, just do it.
2. Gather whatever you can get to support that she told you you did not have to pay the extra assessed support. It may not be much, but it’s better than nothing. At the very least, have it clearly set out when she told, how she told and exactly what she told you.
3. Gather whatever evidence you can that the children were not with her 100% of the time. Photos, receipts/credit card statements for activities with the kids, information about dates they were with you, where you took them, testimony from people who saw you with them etc.
4. So long as you’ve already reported your ex to the government, write to her solicitor stating you dispute the liability, and that the assessment is incorrect on the basis of both the care percentage and the income calculation. Do not threaten to report her, do not mention you have reported her, and do not do it afterwards. State that you had the children in your care for X% of the time, that you are aware she was earning income that was not factored in the calculation, and that it appears she did not inform CSA of either of these things. Further state that she agreed with you that you could continue paying a lesser amount in line with a correct assessment had these issues been considered.
5. Await a response.
Correspondence of the type mentioned in paragraph 4 must be extremely carefully written. You want to dissuade something from taking legal action against you, in circumstances where (on the face of it) it appears to be an easy win. I will almost guarantee her solicitor knows nothing about the issues you raise, and is unlikely to readily accept you are doing anything but trying to throw baseless allegations to get out of it.
This is not the sort of thing you can usually do yourself; especially item 4. While it’s no certainty that it will be accepted in any case, you’re more likely for her lawyer to read it with an open mind if it comes on a law firm’s letterhead than if it comes from you.
I would not raise the issue of custody in the same correspondence as the debt issue. It’s a separate matter, and it is unlikely her lawyer will have any instructions with regards to it. Further, you don’t want to pollute or confuse the two issues.