Despite calling time on their relationship, the pair appear to have chosen to remain close and were recently seen having lunch together in LA.
Dianna and Thomas first started dating in March
The 28-year-old blonde first started dating 25-year-old Thomas in March and the following month the pair were seen looking loved-up at the Coachella Music Festival near Palm Springs, California.
Dianna and Thomas's relationship reportedly got "very serious" in a short space of time, another source told the American publication.
Georgia-born Dianna visited the star in his native Australia over the summer, where she met his family and friends, although she chose not to officially comment on the trip.
While Thomas previously dated actress Lily Collins, Dianna has been romantically linked to Dave Franco, Alex Pettyfer, and Sebastian Stan.