We had a similar issue a few years ago. Jimmy is right, the owner needs Council approval for what they are doing.
I suggest that to save a lot of time you contact your Council and tell them what is happening, and they can require the owner to stop the work. That is what we did, the Council came out pretty much straight away, and issued a stop order. Note that undertaking works without Council approval is a breach of the Environmental Planning & Assessment Act.
The advantage to that approach is that it is then up to the owner to sort out the mess, which would include applying for Council approval which requires the consent of the Owners Corporation. And at least at this stage you don’t need a lawyer although it would probably be a good idea to use one down the track.