when you claim "child care benifit", you need to meet family income limitation.
childcare tax rebate is different. no matter your income, you can always get 50% "out of your pocket" (meaning full payment minus centralink payment).
say you paid 8000 for a full year, centralink helped 3000 (Childcare benifit). you paid 5000, then you'll get 2500 back. (you actually paid only 2500) . or if you paid a full year and centrelink give you nothing, you'll get $4000 back. (you actually paid 4000)
the only limitation is that you and your partner have to work or study or in training.
that is my understanding, anyway.