Many unit blocks in Sydney ban letting car spaces to non-residents and a by-law forbidding it being the most obvious mechanism for doing this.
Also, some blocks in certain local council areas are prevented from doing this under the terms of their Development Approval. Another less direct by-law would be one demanding that owners don’t compromise the security of the building or that their ‘guests’ are accompanied at all times.
However, this owner would indeed be morally entitled to demand that the EC fix visitors parking first before they came after her. Two wrongs don’t make a right but she would have a pretty compelling argument to take to mediation if she was issued with a Notice To Comply.