The Karewa gas discovery highlighted the potential for commercial gas accumulations in the shallow Miocene-Pliocene stratigraphic horizons, thus changing the exploration strategy in Taranaki Basin, New Zealand. The gas origin, its entrapment mechanism, and the petrophysical characteristics of the Miocene-Pliocene Mangaa C sandstone reservoir have been evaluated. Organic geochemical analyses of Mangaa and Giant Foresets shales bounding the sandstone reservoir point to their immaturity (Tmax< 426 °C) and poor capability for thermogenic hydrocarbon generation. Karewa gas displays an isotopic pattern of δ13C1 ≫ δ13C3 > δ13C2 reflecting its biogenic origin with no signs of thermogenic input. Additionally, the depleted δ13C1 (of range −61.32% to −61.02%) and δDC1 values (of range −183.5% to −182.3%) support its microbial origin likely by bacterial carbonate reduction and degradation of the organically-bound CO2 to form methane. Shales of the Giant Foresets and Mangaa formations contain, and therefore contain abundant organically-bound CO2.Karewa prospect as a roll-over structure bounded by listric faults forming a four-way dip closure. These faults are the gas migration paths from Mangaa and Giant Forests shales to the Mangaa C sandstone reservoir. Excellent petrophysical reservoir characteristics of Mangaa C sandstone (visual porosity av. 20.7%, helium porosity from core av. 30.2%, shale volume av. 6.4%, and water saturation av. 34.5%) prompted its gas potential with high hydrocarbon saturation (av. 65.5%). Mild mechanical compaction and low content of authigenic mineral phases aided in the preservation of the primary intergranular porosity. Seal capacity analysis of the Mangaa C system confirmed the efficiency of the overlying mudstones to seal a thick gas column (up to 150 m) with a minimal risk of leakage.