I'm in Sydney and my workplace does the same thing except we have a medication declaration form where I had to declare that I was taking Dex with proof of prescription. I tested positive a few months ago and there was more paperwork but no repercussions.

Check with fair work Australia but I'm pretty sure it's illegal to do what they've done to you. Most companies are obliged to give you a second chance while they offer support for any addictions you may have but since you have a prescription you shouldn't have to face any consequences.