Form 888 (current design date 08/26) is how people who know a couple describe the relationship to Home Affairs, which uses the statements to assess its social aspects. The person must know the applicant and their partner or fiancé(e) and the history of their relationship, and be at least 18. They do not have to be an Australian citizen or permanent resident; evidence of that status is provided only where it applies.
The partner and Prospective Marriage visa pages ask for statements from 2 witnesses, and the form warns that up to 3 may be requested. It is a supporting statement signed as a declaration, not a statutory declaration, so no Justice of the Peace is involved.