I think the one you're talking about is called the FTB reconciliation amount.

Basically it's $600 per child that's held back from your yearly FTB to cover any accidental overpayment. If you end up with a debt then they take out of that first. If no debt you get it paid after you do your tax return. I always over estimate DH's income and get it paid plus anything extra that I don't get in the fortnightly payments.

HTH