Just to say I am not a financial advisor, but I do specialise in this topic. It's my industry and I hold a management position.
It entirely depends on a few factors, novated leasing does reduce the tax you pay but not your gross annual income, so technically child support is worked from gross annual income (before tax). It does alter your tax return so you look like you earn less but not always.
There are two methods of packaging stat method or ECM, a lot depends on reportable FBT. It also depends on if you are full FBT or a charity not for profit or public benevolent etc, etc. using your cap has greater implications.
Reportable FBT will increase income and then increase child support.
But even if you use ECM (which has no reportable FBT), you are absolutely meant to add all GST and Tax savings to your child support estimate, to pay based on your gross income, not doing so could leave you open to legal issues.
I'm not a fan of telling people how to cheat their children out of money they are entitled to, so basically that's all the information you will get from me. At the end of the day, pay for your child. Child support barely covers a quarter of actually raising a child. Please love your child more than you hate their primary caregiver!