Am I paid illegally??

I am currently 19M and in uni. I have a construction job where I am basically treated like a casual. Some days I will get told the night before to not come in and won't get paid and then some weeks I will get to work everyday if there is work (e.g - in uni break and they need work everyday). Also if we were to finish early I wouldn't get a full days pay, etc...

Basically I am just on standby and get to work when they need. It is fairly consistent work but nonetheless still feels casual.

However my drama is I only get paid $20 per hour?? This just feels way too low for what I am doing and I work incredibly hard at the job and go above and beyond to do well at it. It feels low because of the casual aspect but not being paid a correct casual rate? Unless I'm wrong?

I am trying to find a new job but struggling atm, anyway is this even legal pay? And is it normal? Feels really low for how hard I work.

P.s - I am basically a labourer but have really built up my skills on the tools over the last year or 2.

Any suggestions on what I should do?