For goodness sake people, why all the hand wringing.
"I cannot undertake my rostered duty as I am fatigued, it would be unsafe and unlawful for me to do so."
(choose any one) "you are not fatigued" "you are malingering" you've just had the minimum requied rest, so what's your problem" etc. etc.
"Just to clarify and so I get my Air Safety Report correct, your name is ......., your position is........... I have just advised you that I am unfit for duty and you are advising me that I must operate unfit. Is that correct."
If Yes: "I advise you I will not sign on for this duty, this conversation is now ended."
The solution is in your hands. If they pressure you to do unsafe things you are better off not being there. If they dismiss you, you will get heaps on a wrongful dismissal.
What is the problem girls.
Maui