It will be in the by-laws … or not! If there is a by-law stating what you may or may not have on your terrace then that is the only basis on which you can be “breached”. Your notices to comply should state the terms of the by-law that you are alleged to have breached.
If they don’t have a by-law, tell them to leave you alone. If they do have a by-law, but its terms are being over-reached, let them take you to NCAT for fines and then challenge it there.
Either way, if the situation is as you say, tell them to do this through proper channels or leave you alone