JC – the Rental Agent is a Member of your Plan’s Executive Committee (E/C) because he was nominated, seconded, and elected by a majority of those who were entitled to vote at the last Annual General Meeting (AGM), and then elected Secretary at the following Meeting of the new E/C.
You mentioned in your first post that this individual exercised an Owner’s proxy to vote on the submission of a Development Application, but that’s only one vote, so a majority of the others who were present at what I assume was a General Meeting (because there a no votes by proxy at E/C Meetings) must have voted similarly.
So it’s not the Rental Agent alone who’s to blame (if any is to be apportioned) for the submission of the DA, but rather it’s the Owner who seconded his nomination and the majority of Owners who then elected him to the E/C.
You and like-minded Owners could correct that situation at the next AGM, but in my opinion there’s nothing additional to the actions that you or your Owners Corporation is taking with the Department of Fair Trading, with Council, and via a Special By-Law that may be reasonably taken, as under the provisions of the NSW Property, Stock, and Busines Agents Act that governs their activities, neither of you is a “client” of the Rental Agent and so you have no ability to complain.