Seems like a reasonable direction from the employer.
However there is no law that says an employee must sign. To force an employee to sign under threat of dismissal is a form of duress and likely nullifies the signature.
That being said, most employees have a condition in their contract that says they will follow the employer's policies so strictly speaking a signature is not required.
If there is something contentious in the policy take it up with the union. The staff code of conduct needs to be reasonable in its own right and can't change your employment conditions.
Practical tip: Sign and don't make things difficult in the workplace.