I agree with Whale. A plan of subdivision should have been done to reflect what happened, as the strata plan would be incorrect.
The other factor is that if OC approval wasn’t obtained, then I would say it is highly likely Council approval wasn’t obtained, and it is very probable you would need Council approval for that kind of work, i.e. removing a wall between two units. I suggest you give the Council a call and if necessary, go there and do a search to see if any DA was put in and approved. If Council approval was not obtained, then at the very least retrospective approval needs to be given. The Council would have to be satisfied that it is structurally sound and the integrity of the building is not affected. At the extreme the Council can order the wall to be put back, although I am not suggesting that.
Note that when someone puts a DA application in to Council, it usually requires the common seal of the Owners Corporation.