If it is common property then you cannot put anything there. If not, there are usually bylaws about changing the external appearance.
To actually take action they would need to go to NCAT. It would be a waste of time for everyone because all that would happen is your brother would agree not to do it again.
The most important point is does he really want to annoy his landlord. At the end of his lease might he want to extend the lease. Maybe the landlord will find a way to make life difficult. Is it a small town and the landlord will make sure everyone knows that he is a difficult tenant. The easiest and most sensible thing to do is to dismantle them. If he wants to he could apologise for not checking with the landlord first.