It's not dollar for dollar your friend is losing. It just appears that way at the moment

If you are supposed to have 50 a fn deducted (as an example) for 26 fn for your normal entitlement, but you get zero deducted for, say, 20fn. You are a thousand dollars "behind". If you then receive your "normal" entitlement, you have 6fn to deduct your 50 per fn, (normal) plus the thousand dollars you are behind - so another 167 per fn - so your total deduction just to catch up is 217 per fn for the balance if the year. Just to catch up. Then you have arrears so you actually have to factore in a bit extra as well for every fn through the year

Now, with the method if calculation you are using, the system also looks at your most recent payment (for you, being 6k) and assumes you will get that again next month and every month til the end of the year, so it will likely cut your payment drastically, possibly completely. Next month when you get nothing, it will all be recalculated based on you getting nothing and the system assuming you get nothing each month.

It is messy. That's the risk with that method of assessment. And you aren't going yo be able to anticipate payments. In advance. You just have to wait for the letters to come in. And you need to budget around it by not spending the child support arrears. Technically speaking you have been overpaid every fortnight this year so even though it feels like you are getting child support arrears and should be ok to spend it as you would have previously if you got it fn by fn, you have to be careful with it while your FTB is sorted - otherwise you will end up in a big financial mess