OK, Ive just been on the ATO website to check.
There are no parial payments, you either get 900, 600, 250 or nothing depending on what you income was, and you need to have paid $1 in tax because they always round down.

There is a calculator on there that will tell you if you are eligible, and how much for, so as a test I fudged some figures and said i earned 30K, and made it so my tax liability was $100 -- it said I would get $900 based on that info.