Before I help you think about the ways in which the mandala motif reflects diasporic Australian identity, I think the terms motif and diasporic should be clarified.
A motif is a central theme or idea in a book. In Patrick White’s novel The Solid Mandala, you might argue that one of the key themes involves wholeness or harmony. That theme could connect to diaspora. Diaspora refers to people who’ve been dispersed from their country of origin. It might be kind of hard for someone to feel whole or complete if they’re separated from their homeland.
The tension between diaspora and completeness could be symbolized in the mandala. Perhaps you noticed this quote near the end of the novel: “The Mandala is a symbol of totality.” Throughout the book, it seems like Arthur tries to attain a kind of totality. You might see it in the flashback of Arthur on the ship with his family. Remember, Arthur comes close to falling off. When his mom tells him that he might fall into the water and be lost forever, Arthur replies: “Yes. I might. For ever.” You could argue that the appeal of falling into the sea and getting lost “for ever” has something to do with Australian diaspora. You might argue that it’s tied to his longing for a permanent, stable home. Remember, Arthur and Waldo weren’t born in Australia: they migrated there from England.
You could also discuss the mandala and diaspora in the context of the marbles. Perhaps Arthur’s “silly old marbles” aren’t so silly. Maybe they indicate the mobility of Australian/British identity. No matter where a person is, they can still carry their homeland/ adopted homeland with them in a figurative or symbolic way.