'Dissertations and theses are a subcategory of grey literature, which may report studies of relevance to review authors' - Cochrane Handbook, 1.1.5 Dissertations and theses databases#section-1-1-5
Research degree theses and dissertations can be another source of unpublished studies.
The Library's Theses guide focuses on two major sources, Trove (the key search tool for discovery of Australian theses) and ProQuest Dissertations and Theses (a major commercial database with millions of theses, primarily from the USA but also many other nations).
See the database list for Theses for links to these and other sources.
'Specify all databases, registers, websites, organisations, reference lists, and other sources searched or consulted to identify studies. Specify the date when each source was last searched or consulted' - PRISMA Explanation and Elaboration, p. 5