We have had a similar issue, and have the toilet door code displayed on the Notice board (located inside a foyer, so only visible to residents and visitors in our security building).
But what you are doing is problematic under strata law. The toilet is common property, so all owners and residents have the right to use it equally. You can set rules (eg, door code only available to workers on request), but those rules must be in a by-law registered and updated as a special resolution at a general meeting. You cannot legally discriminate against some owners, or just give a small group of owners rights that you deny to others. In strata, it’s all in, or all out, no exceptions.
Similarly, shutting down an asset of the building, by closing the toilet, requires a special resolution at a general meeting of all residents. A special resolution needs a majority of owner to vote for it, but fails if a quarter (ie 7) owners vote against it. You might be lucky, to not get 7 owners not to oppose the motion, but in the future some owners will get annoyed to see a toilet they can’t use, and will agitate for its reopening. Note that if you have a swimming pool or gym or the like then you might need to make a toilet available under other legislation, I believe. You development application might also specify the toilet is required…
My suggestion is to create a by-law for its use. Such as the door to have a coded lock, the code to be changed yearly, and to be made available to the strata manager, office holders of the strata committee, and any owner who requests it, with a good reason. Or perhaps to display the code on the building notice board only visible to residents. Also, this is a good opportunity to consider installing CC TV cameras. We have them and they record the common area outside the toilet, so anyone using the toilets is recorded. A notice that there is surveillance, will improve resident’s behavior generally, in our experience. Plus it can be used to identify vandalism, if you are lucky with catching when it occurs.