As far as I know, they take his income from the ATO from last financial year, when he put his tax return in, work out what his payments would be from that & thats what he is to pay this financial year unless he disputes it & gets it dropped. I got them to drop it for DH when we were seperated, but that was my decision. Not sure if they can do it themselves.
If he doesn't pay you enough, as far as I know, when he puts his tax in & they see the amount he actually earnt they can take his tax pay out & give it to you. I think you need CSA to be collecting for that though, not a private arrangement.