Just got put on casual.....
Not good to hear and no they can't do that. With warnings, it must be 3 written warnings for the incident in a 12 month period before getting the flick unless the incident is criminal offence.
Go to consumer employment protection / fair work and lodge a claim
Also if you become a Union member now, it wont be backdated or retrospective cover, industrial representation usually needs a minimum of being financial for at least 12 months prior to the incident, well it used to be when I was a rep but it may change from union to union
Never underestimate the power of the darkside, let the tap flow through you.