cookie - for what it's worth, if a couple gets back together on centrelink files, it should transmit to child support - so if letters are still coming out from child support, it is very suss.

as i said earlier, you can do an anonymous tip just saying you're aware that they are living together, but have had mail sent to your address from child support. IF they've done the right thing, nothing will happen - the tip off will be closed, with no action. if not, they SHOULD be investigated.

i would word a tip off in those circs something along the lines of:

previous residents of this address have not had their mail redirected and mail has been received from child support agency which has been returned to sender. previous residents are people that we know, have been living in a marriage like relationship, and children are biological children of the couple. i'm unsure if the mail relates to a time when they were seperated and they have simply forgotten to notify a change of address to child support, but this has concerned me and i wanted to pass on the details so that, if there is something out of line on their record, it can be investigated