The assembly of island plant communities is the result of a number of processes: immigration (dispersal), speciation, and extinction. Using four plastid genes and one low-copy nuclear gene, we investigated the origin of the New Caledonian Grevillea (Proteaceae), an otherwise largely Australian genus. In the combined plastid analysis, the species form two distinct clades, the exul (four species) and gillivrayi groups (six species), within group 3 of Grevillea. All New Caledonian Grevillea display two distinct copies of PHYA, one copy in group 3, and another in group 4. Previously published chromosome counts for G. meisneri and two new genome size estimates of G. gillivrayi and G. rubiginosa suggest that these plants are tetraploids. Altogether, the current data available suggest that New Caledonian Grevillea are allotetraploids, resulting from one or two hybridisation events between two or three distinct parents. A possible scenario is that modern Grevillea has descended from a hybrid swarm that formed 9-13 Mya between multiple immigrants (now extinct) that reached the island. Grevillea can be added to the list of island plant radiations with an early history of hybridisation and polyploidy. The relative importance, location and timing of these two mechanisms remain to be elucidated. The two groups of New Caledonian Grevillea may be easily distinguished by their inflorescences, flowers and fruits, with the gillivrayi group displaying greater ecological and morphological diversity. The seed characteristics of New Caledonian Grevillea are consistent with their pioneer behavior.