Being underpaid, and forced to work unpaid hours?

I am a 20 year old working in a restaurant in Australia. Previously, I was working at another restaurant as a casual waitress and being paid 16 dollars an hour before tax on weekdays, 22 on Saturdays and 28 on Sundays, which I believe is correct. However, the owners of that restaurant moved away and the business was not sold (I live in a very rural area, it's a bad market) and I thus started working at another restaurant 10 minutes away with encouragement from a friend. At first, it was fun. I was being paid only ten dollars an hour on weekdays and weekends, but I assumed it was because I was new, because I was now part-time and not casual, and that everybody was being paid the same. But then the inconsistencies began to crop up. Firstly, I applied for the position of waitress, but was also expected to do the duties that are normally designated to busboys, kitchen hands and hosts. Okay, I though, they want to make the most of us. Then when we were asked to fill in our tax declaration forms, we were told to check 'casual employment'. And then, while Australia is not a tipping culture, any tips we did receive from tourists or kind locals were not given to us. Then a big problem started when we were being forced to 'sign out' (we wrote our time of arrival and end time in a book, and signed it) half an hour and sometimes an hour or more earlier than we'd actually finished. Our boss said it was because we were 'just hanging around talking' and maybe the other waitresses were, but in the meantime I was finishing all the work including the dishes, clearing and resetting tables, etc. Yet I was also made to sign out much earlier. The final straw was when my friend from the same restaurant and I, we are the same age, went to cash our cheques. Although I had more experience than her and required less training, and although we'd worked the same hours precisely, she was being paid 14 dollars an hour and I only 10. We confronted our manager about this and were both put on 14, but considering it's been well over six months and we are capable of performing every necessary task carefully and quickly, our rates have not been raised to what other casual waitresses should be receiving, and our rates are the same on weekends although we work on Sunday. My partner wants to move interstate and I am tired of being paid so low to work so hard (we've had to replace our manager recently and were also expected to do his duties). I like my boss as a person, but I feel that this is unfair especially as we cannot make up the difference through any tips at all. I'd like to quit, but was wondering if I was legally entitled to any back pay?