Formal letters purporting to represent the Owners Corporation should be approved by the committee, not issued unilaterally by the secretary or treasurer.
That said, as a treasurer, I occasionally sent an informal and friendly reminder/explanation note about levies before the more formal correspondence from the managing agent would be received, particularly if the owner was new and might not understand the process, and I did not always run that by the committee. However, I did report to the committee on how many owners were behind on levies and I did tell them whenever I had sent such notes.
I don’t think NSW has a code of conduct for committee members but in the ACT, creating a nuisance over petty matters while purporting to represent a decision of the committee (which exercises the functions of the owners corporation) would be contrary to the code of conduct.