Strictly speaking, it should have a DA as it is changing the layout of the apartment. That said, we had a real estate agent living next to us who took out the bedroom next to his lounge room so he could throw bigger parties (his words). That meant that instad of having our bedroom adjacent to his bedroom, it was no adjacent to a party room.
When we complained to council, we were told it had been signed off by the builder as a “compliant development” meaning it didn’t need a DA. The strata committee couldn’t refuse it and neither could the council. Our advice was to sue the builder. We sold, instead.
So my advice would be to notify council that there is a non-compliant renovation in your building and then let them do the heavy lifting.