I am not in NSW and not a lawyer but it would seem on the face of it that you have answered your own question. If the Act says a building manager cannot be a committee member and if the general meeting appointed this person to be a building manager and then conducted an election for committee members, then it would seem to me that this person was not eligible to be elected as a committee member so their election was invalid.
I think the EC could be advised that the person can continue to be building manager but cannot continue to be a committee member. The committee will need to appoint another one of its members to be treasurer.