AidanS: Thanks for all the responses guys!
I'm going to write a letter and see how things go. As I said before, when ANZAC day popped up on a Saturday a lot of the "casual" (my employer has no part time contracts, only full timers and casuals which are typically students) were legally entitled to that Saturday lieu day, as they work it every week, but the employer sent out an internal notice stating that casual staff only get the 1.5x pay rate and that's it.
I challenged it, and a few of us got lieu days. But most of the "casuals" (casual means regular weekend worker in 95% of cases in this company) were oblivious to their entitlements and trusted the information their employer was passing on to them (as anyone probably would!).
This sick day situation is now apparent to be the exact same thing. A tad disappointing considering this particular company has been an employer for 50+ years. Would be interesting to know how many $$$ they've saved from preying on the ignorance of students.
Casual should mean just that, someone without fixed hours that are asked to work as cover for someone who is sick or at busy times. The moment you start having fixed shifts, like every Saturday, you are no longer casual.