fencing - NCAT
If they do serve a fencing notice on you then you and your neighbours can take it to the local court or to NSW Civil and Administrative Tribunal (NCAT) and have the matter heard.
Going to NCAT for a decision is generally faster than the court (they try to hear the case within 4 weeks) also they are not bound by the rules of evidence so it is less formal than the local court – they do require that all parties involved try to mediate the situation first.
Note that when the court decides what a “sufficient dividing fence” is they take into account
The standard of the existing fence;
The purpose of the fence;
The way the land on either side of the fence is used;
The privacy or other concerns of each neighbour; and
The kind of dividing fence that is usual in the area.
So if the existing fence is metal/colourbond and in good condition (apart from the damage they caused) and the common fence in the area is metal/colourbond then there is a good chance that the “sufficient dividing fence” will be deemed to be metal/colourbond. If the fence is 120/30mm on to your land an you don't object to it then there is logically no reason for it to be replaced.