Does the land grab disadvantage any other residents? While common property, it is property that nobody else would or could have used?
My concern would be 1) that the area is unavailable to others for their use and enjoyment and 2) that the Owners Corporation is liable for anything on the common property that it has not been maintained and should have been maintained. What if there is something dangerous that can’t be seen behind the fence? What if the fence falls over on someone?
If nobody else wants to use the area, then a solution could be to pass a bylaw (Special Privilege Rule in ACT-speak) giving exclusive use of the area to the relevant unit owner on the condition that the owner of that unit at any time must undertake any necessary repair and maintenance within the area and of the fence. That permission could be granted subject to some one-off payment or an on-going annual amount.
If something was not done properly in the past, the situation can be rectified by doing it correctly now.