Keep in mind that you get an amount of child support that doesn't impact your FTB (around 2900 for you with four kids) and then you are impacted by fifty cents for every dollar. So out of your 6k, the first 2900 ish doesn't hurt your payment. Then only half of the remaining amount (3100 ish) so 1550 would be what you lose from your FTB for the full year.

It sounds like this is what you have calculated if your are factoring a loss of $100 a fortnight over five weeks as a third of what you think you should lose. It is possible that the CSA hasn't sent across full amount at once if you haven't yet received it all.

The other thing is that mandatory adjustment to your payment doesn't occur until the day info is received by centrelink so it may not apply to your full payment this fortnight.