Where materials that you are referencing cannot be retrieved by someone reading your work, they should be referenced as personal communications. Examples of personal communications include:
Personal communications appear as an in-text citation in your work, but are not included in your reference list.
A personal communication is indicated by writing personal communication immediately after the name of the person who provided the information. The person's initials are also included, which is not normally the case for in-text citations.
Australia's banks are strongly positioned to manage corporate governance issues (J.R. Weissmuller, personal communication, April 8, 2020)
J.R. Weissmuller (personal communication, April 8, 2020) advised that....