If you were casual and around 16 - 18 I wouldn't take less than $10/hour casual, I'd be chasing $12-13/hour.
Based on that the amount a bank would loan you depends on your ability to service the debt.
If you earn $15,000/year (For example on a casual wage or something) you of course need some living money (food, board/rent) and then you need to work out whether you have enough for loan repayments and insurance, plus savings money incase something happens.
On $20,000/year I would say a bank wouldn't loan you much more than $10,000.. That's really only a guess though.