When you say Agent, do you mean strata manager?
You need to go to Fair Trading to apply for orders to restore common property (the doors) to their former status.
You might also need to get interim orders tell the owner not to change the other doors.
And you could write to council saying this owner has changed the layout of the apartment (which requires a DA).
If the owner refuses to change the doors himself, send workmen in to do it and then send him the bill.
And when you have sorted all that out, sack the “Agent” who is taking your money and not even doing the basics.