Well thats not on.
I would tell your Mum that either she contributes to her upkeep or you will be ringing centrelink to let them know the circumstances have changed. Also when they changed so your Mum would possible get into strife for being overpaid.

It does cost alot to try and feed let alone clothe and entertain a teenager and if your Mum is getting paid for the job but not the responsibility then I would be kicking up a stink.
Yes it may cause some grief but you cant feed her on just love!