I would start from the point that Section 139 says any by-law “must not be harsh, unconscionable or oppressive.”
Then, by implication, a bylaw that required committee members to act reasonably, fairly and honestly may well pass muster.
However, if it insisted that they enforce the by-laws, that might be seen as oppressive, as there may be times when it is in the best interests of the majority of owners not to do so (for instance, when informal parking arrangements technically infringe by-laws but every body is happy with them).
Also, you can’t demand that strata committee members sign a code of conduct as that might limit their ability to serve on the committee in a way that that Act does not envisage.
It would be a too easy for someone to sign a code of conduct and just ignore it when it suited them. What do you do then? Issue a Notice To Comply?
I know there are circumstances where having a code of conduct might appear to be helpful, but if you have someone who is going to behave unreasonably, then I wonder if a code of conduct is going to make much difference.
That said, these documents often act, at best, as a reminder to people of how they should behave rather than a mechanism for punishing them when they don’t.
It’s like a mission statement – this is how things should be (even if they often aren’t).
On balance, I think a Code of Conduct like the ACT’s might be valuable but I wouldn’t go to the barricades to have it introduced.