As I posted on the other thread - get it in writing from the company before you take the tablet, that taking a Codral is not going to result in your job, even if tested.
If the company won't come to that, then you call in sick as you take the tablet. That gives the company time to find a replacement crew. How easy or not that is for them is not your problem - and when enough people do this, the company will start lobbying CASA to get codeine off the prohibited list.
If the company happens to be Qantas, CASA will immediately remove codeine from the list. If the company isnt Qantas, well, nothing will change.