I think the safest thing for an OC in a state or territory without a legislated code of conduct would be to adopt verbatim the code of conduct from one of the other state’s or territory’s Act. It could be adopted as a by-law (Aka article or rule depending on the jurisdiction) that directs the action of any future committee member. A committee is bound to act in accordance with the direction of a general meeting resolution (unless it would be illegal).
That would get around the discouraging business of asking/requiring individual committee members to sign a bit of paper.
It would also reduces the risk of a home-grown set of words that it might be drafted with some non-obvious flaw. A code of conduct included in another state’s legislation would have survived considerable scrutiny. It might also be more readily supported by conservative owners for that reason – less risk of some unintended effect.
If a committee member then failed to act in accordance with the code of conduct, you have the option to invoke the processes for a rules infringement/breach of by-laws. Following a process set out in legislation is more likely to be upheld if challenged than some home-brew concoction.