You're in Australia, right? Well, I'm not sure how it works there, but for the US this is what it's like:
I know someone (an ex) who is a commercial diver. He enjoys it but it's very difficult (not to mention very dangerous) work and it takes a LONG time to get to the point where you're making really good money with it. Usually after training you get shipped off to an oil platform in the Gulf of Mexico to work as an intern, *maybe* getting into the water once a month if you're lucky... you put in your time doing that for a year or two at around $10-$15 per hour and then you'll start doing more serious work that pays better. That's the case for almost all commercial dive school grads in the US.
But like I said, I don't know how it works in Australia. Good luck!