Right to Privacy of Terrace

We have owned an apartment in Potts Point for 6 years with a large private terrace. The apartment adjacent to us has recently been sold with “exclusive use” rights to the rooftop area outside the apartment purchased by the previous owners from the building developer, and a by-law was created granting the rights to the space. Up until now this hasn’t been a problem but the new owners want to install balustrade fencing around part of the roof area which will be visible from our terrace and anyone standing next to the balustrade can look down onto our previously private terrace and into our bedroom and living room. The owners corporation (which the new owner previously chaired) has granted approval but we assume they will have to lodge a DA for the construction. Is this correct? Can we prevent them from building onto their roof area effectively eliminating our existing privacy? What are our rights in this situation? Any advice much appreciated.