Have a read of the Sydney Airport Curfew Act 1995.
- Breaking the curfew IS a criminal offence.
- YOU as the captain can be held legally responsible.
By making the decision to depart after curfew, you are assuming operations staff are fully conversant with the law, that the company will be willing to admit liability and that the authorities will only take action against the company, and not the Captain as well. Unless I can answer all of these with 100% certainty, I ain't going.
FWIW, Ops have led me up the garden path plenty of times. They are an excellent source of information, but at the end of the day it's your licence, not theirs.