From a basin-wide evaluation of organic geochemical data, it has been possible to characterize and differentiate various source rock units and to establish the genetic relationships of oils. Most of the oils are primarily terrestrially sourced, and it is possible to recognize varying contributions from source rocks within the Paleogene Kapuni Group and late Cretaceous Pakawau Group. This distinction is attributable to the rise to dominance of angiosperms over gymnosperms in coastal plain swamp communities by the Eocene. Apart from the Maui family (i.e., Maui field, Maui-4, and Moki-1) oils, the inferred relative contributions from the main source rock types generally correlate with the relative proportions of suitably thick and mature units near reservoirs, given that mos Cretaceous-sourced oil appears to have escaped prior to trap development. Maui family oils do appear to be primarily sourced by Rakopi Formation (late Cretaceous) coals. In the northern part of the Taranaki Peninsula, where heat flows are highest, Mangahewa/Kaimiro Formation (Eocene) coals are the chief sources of oils. Farther south, in the Kapuni and Kupe South fields, Farewell Formation (Paleocene) coals appear to be the main oil source rocks. Biomarkers suggest that the onset of oil expulsion from coals occurs at a maturity level corresponding to a vitrinite reflectance of ca. 0.8% R_o, and may be aided by the evolution of large volumes of carbon dioxide. The terrestrial influence on Paleogene source rocks diminishes to the north-northwest of the basin and increasing marin contributions to oils are observed. A late Paleocene marine black shale is the source of oil in the Kora volcanic structure. It is possible that shales interbedded with coals, reflecting periodic marine incursions of coastal flood plains, also contribute to oil generation throughout much of the basin.