Owner altering common property – how to stop him?

Hi Gurus

Query is, who to turn to when an owner goes ahead with a threat to move the front door to his unit (which is common property)?

His request to do this was voted down at the AGM in November 2018, mainly because he has already taken over common property on that floor by blocking off access to the common area laundry and a section of the hallway, via a second locked door he installed across the hallway. He has been asked to remove this second door but has refused. The agent won’t do anything.

The owner now says he is frustrated with delays and is unhappy he has been knocked back, can’t see why anyone else cares, and so will go ahead with his plans anyway. So, if and when he does, who can the Committee turn to to stop it?

Is it the local Council, even though this alteration is interior to the building? I know Council will order reversal of unapproved outdoor constructions which are visible, but this particular plan with doors is interior to the building.

Who will be backup to stop the works if and when they begin: should we turn to NCAT, or first to Fair Trading? Or even the police to stop unapproved works? Our agent will do nothing, as usual.

Suggestions? We want to be prepared with our own realistic threats, or he will get away with it. We do not want to go to court.

Thanks!