Sir Humphrey is right. In terms of NSW law and generally, the issue is that there are certain powers that are reserved to the owners, they can’t be “given” to the Committee. That is – for numerous matters the only proper exercise of power is by the owners voting at a general meeting, either by ordinary or special resolution, the latter requiring 75%.
This includes matters such as changes to common property.
So in this case it is not a matter of revoking whatever power was purported to be given to the Committee, because you can’t give that power to the Committee. If the Committee is making decisions such as changing common property without the approval of the owners voting at a general meeting, then they are acting ultra vires (outside their power).
I suggest you contact the strata manager at first instance and ask them on what grounds they believe that the Committee is acting within their powers and in accordance with the Act. It is likely that the manager is also clueless, in which case you will have to escalate the issue. The thing is, if you put the Committee on notice that they are acting outside their power/authority, and they continue to do so, they can be liable for those acts.