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We present a molecular phylogeny and taxonomic review of the Pacific island colymbetine diving beetles, focusing on the Fijian and New Caledonian faunas. Four new species are described: Rhantus monteithi and R. poellerbauerae from New Caledonia, and R. kini and R. bula from Fiji. We also describe the 3rd instar larvae of R. monteithi and R. poellerbauerae spp. nov., assigned to adults using mtDNA sequence data and discuss larval characters in the light of phylogeny. The phylogenetic hypotheses derived from both parsimony and Bayesian inference based on 3508 aligned nucleotides from a combination of mitochondrial ( cox1, cob and rrnL-tRNALeu-Nad1) and nuclear genes (18S rRNA and H3) reveal a clade comprising R. novaecaledoniae, R. alutaceus, R. pseudopacificus, R. monteithi sp. nov. and R. poellerbauerae sp. nov., which agrees with the R. pacificus group sensu Balke (1993). Carabdytes upin was included within this clade, possibly indicating paraphyly of the genus Rhantus. Rhantus annectens, R. bacchusi, R. supranubicus, R. suturalis, R. simulans, and the Palearctic R. exsoletus, R. latitans and R. bistriatus formed a clade corresponding to the R. suturalis group sensu Balke (2001). Rhantus vitiensis, previously assigned to the R. pacificus group, was included in the R. suturalis clade. We find some support for a scenario where the Pacific was colonized out of the Northern hemisphere only during the past c. 12 million years, rejecting a Gondwanan origin of the morphologically isolated endemics. The new species are all characterized by mtDNA haplotype clusters, the degree of divergence between sister species pairs ranging from 1.3 to 7%, while R. novaecaledoniae individuals from all over New Caledonia apparently form one morphospecies, with moderate genetic diversity (up to 2.3% mtDNA divergence between populations). The sisters R. pollerbauerae sp. nov. +  R. monteithi sp. nov. occur sympatrically on Mont Panié but appear ecologically segregated, while the sisters R. vitiensis +  R. bula sp. nov. were encountered syntopically on Viti Levu. Comparing genetic and morphological data of Fijian Rhantus and Copelatus diving beetles, we here show that even in island radiations it is not per se possible to know if mitochondrial DNA barcoding would perform well ( Rhantus: YES, Copelatus: NO). At the same time we show that fixed cutoff values, as sometimes used to discriminate between barcodes, thus species, might be meaningless. We underpin the importance of morphology for sustainable exploration of global diversity. [ABSTRACT FROM AUTHOR]