A distinctive organic-rich marine mudstone of Late Paleocene age occurs in most of New Zealand’s sedimentary basins and has been identified as a potential source rock for oil and gas. Identified as the Waipawa Formation in the East Coast Basin and the Tartan Formation in the Great South and Canterbury Basins, the unit is a relatively uniform massive mudstone that varies greatly in thickness (2 - 70 m) and grades laterally into distinctive facies equivalents, notably greensand and a thin-bedded siliceous mudstone. All these facies are characterised by relatively high TOC (0.5-10 wt%) and 13C enrichment (δ13CTOC > -24‰), and we refer to them collectively as “Waipawa organofacies”. Our detailed stratigraphic and geochemical studies refine the age (58.7 to 59.4 Ma), distribution and nature of the Waipawa organofacies. We have determined that deposition occurred in continental margin settings throughout much of the southwest Pacific under cool, dysoxic conditions associated with a significant influx of terrestrial organic matter, high marine productivity, a global fall in sea level, and a regional unconformity across shallow and deep marine settings. The combination of cool temperatures, lowered sea level and bathyal erosion suggests that deposition was linked to short-lived growth of an Antarctic ice sheet in the earliest Late Paleocene (~ 59 Ma).