The distributions of sterane and terpane classes of biological marker hydrocarbons in seven oils and in selected sediment extracts from the Westland basins region have been examined by computerised gas chromatography/mass spectrometry. Despite the moderate levels of biodegradation in several of the oils, there are molecular indicators suggesting the incorporation of higher plant derived organic materals in their source rocks. In oils from the Murchison Basin and from Madagascar Beach, there is evidence to suggest that they are of a mixed marine/terrestnal ongin. Oils from the Grey River Basin have been shown to be derived from sediments that have incorporated largely terrigenous organic materials. Moreover, high concentrations of a single diterpanoid hydrocarbon, believed to derive from plant resin constituents in both the oils and in the extracts of sediments (coals and shales) regarded as being potential source rocks, suggest that the oils and coals were derived from the same comferous plant debns. The possibility that the oils are derived from these particular coals/shales has been discounted from a comparison of molecular maturity measurements on the oils and rock extracts. Based upon these data, the Upper Cretaceous Paparoa coals of the Rapahoe sector are of insufficient thermal maturity to have sourced the oils encountered within these measures. Coals and shales comprising the sedimentary succession of the Card Creek-1 well also appear to be of insufficient maturity to have sourced these oils.