I’d make a couple of points here – the Chairman/Treasurer doesn’t have any special privileges in making up the rules as he goes along. Are there any other members of the Exec Committee? In any case the OC should be taking back control from this person. Ordinary keys can be easily copied and I’d tend to agree with the technician rather than the Chairman/Treasurer.
From what you say, the remote access would have been provided as an integral part of the common property. Under Strata Law the OC has an obligation to maintain that common property (in working order). So the Chairman/Treasurers decision to deactivate that part of the door mechanism without putting it to a general meeting would in fact be unlawful.