The ATO will eventually take him to court to do his tax returns.
You can ask CSA not to pay you because you're worried about an overpayment - you can say I want XX and you have to keep YY in case of overpayment. This is what I'd do if I were you.

The other thing - if you ARE overpaid Child Support, then he would have to take you to Court to get it back (very expensive), whereas Child Support Agency will give him a "credit" on his account - ie if he's overpaid $300 which means he's six months in front, then he won't have to pay for six months. But this will only happen on a reconciliation AFTER he lodges his tax returns.

So spend what you get - he's only going to get a notional credit for it... especially if it's only $50 per week. And NEVER rely on him paying it anyway. They rarely do.

Good luck.