An academic offence is any conduct/act which attempts to deceive or is in contravention of any rules or regulations governing assessment. It applies to any work submitted regardless of its contribution to your final grades and includes but is not limited to written assignments, data collection, exam papers, imagery, music, ideas and thoughts.
If you are focusing your PhD on a topic you also wrote about in your previous studies, it can be tempting to refer to work you have already written. If you are making use of your own previous work, this does still need to be referenced and using it without reference counts as duplication which is an academic offence.
You can read more about support available if you receive an allegation of academic misconduct here.