Elytra yellow with red lateral margin (Barker 2006a), approaching the colouration of C. sexplagiata from eastern Australia.

Until October 2021 this species was known in SA only from the allotype specimen with the general location 'Nullarbor Plain' and the 1984 paratype by M.Powell 32 km E of Eucla, (only c. 20 km into SA from the WA border).

It was noted on these pages, however, that there were specimens which approached this species to some degree amongst problematic material formerly assigned to C. vanderwoudeae from the Flinders Ranges and nearby areas. Some of these have a suggestion of the two-toned elytral background, being somewhat yellow-orange medially with the margins a more reddish dirty orange. These may represent intergrades with C. nullarborica, but they also have similarities with other species in the C. sexplagiata group, and are treated here as a separate entity, C. sp. Flinders Ranges (q.v.).

In late October 2021 Mark Hura collected five individuals of C. nullarborica in SA well to the east of the Nullarbor Plain at Wandilla Beach, 16km W by S from Fowler’s Bay. Soon after, in late November and early December 2021, six specimens were collected more widely in the same region by Ben Parslow et al. on the Fowler's Bay-Yalata Bush Blitz survey.

A single paratype specimen from the same locality as the holotype (5 km E of Eucla, WA) is recorded as being from "Eucalyptus sp.". The recent collections come mostly from white flowered hosts, including Sugarwood (Myoporum platycarpum), but one is from the violet flowered Scotia Bush (Eremophila scoparia). Both these species were formerly classified in family Myoporaceae, and are now included in a broader Scrophulariaceae, but there are insufficient records from the family to establish a clear association. There is evidence of such an association though in two other SA species of the Castiarina sexplagiata group: C. erythroptera is found largely on M. platycarpum in SA; and C. vanderwoudeae is found almost exclusively on E. scoparia.