First of all, please, please find out what award you are meant to be under and call Fair Work. If you don't know your award, they can help you out.

Whilst you can agree with your employer to be paid differently to an award, you cannot be worse off due to it.

I work a lot with awards in various industries, the less pay when you are not supervising, is not actually that suss, but pretty standard. Different pay for different shifts, depending on your responsibilities, typical with casuals. What does seem suss to me is being employed for 40 hours a week as a casual. I have not seen award in the past few years where that is acceptable at all. Casual and part-timers are not meant to work full time hours. Also the standard work week these days is 38 hours, so if you are doing 40, then the chances are you should be paid for 2 hours overtime at least!

Now, don't go after your employers with what I have said above. You need to know what is in YOUR award and understand it so you can have a fair and equitable discussion with them. Write down the main points, the sections of the award that are relevant, even have someone with you if you feel the need. To stand up for your rights, you need to know your rights in the first place.

Best of luck!