The reorganisation of Earth's climate system from the Oligocene to the Miocene was in fluenced by complex interactions between Tethyan tectonics, orbital parameters, oceanographic changes, and carbon cycle feedbacks, with climate modelling indicating that pCO2 was an important factor. Oscillating episodes of climate change during the Oligocene-Miocene transition (OMT) have however been difficult to reconcile with existing pCO2 records. Here we present a new pCO2 record from the OMT into the early Miocene, reconstructed using the stomatal proxy method with a database of fossil Lauraceae leaves from New Zealand. The leaf database derives from three relatively well-dated sites located in the South Island of New Zealand; Foulden Maar, Mataura River and Grey Lake. Atmospheric pCO2 values were obtained based on four separate calibrations with three nearest living equivalents, using the stomatal ratio method as well as transfer functions. Our results, based on the mean values of each of the four calibrations, indicate pCO2 ranging ~582–732 ppm (average 650 ppm) at the OMT, falling precipitously to mean values of ~430–538 ppm (average 492 ppm) for the earliest Miocene and ~454–542 ppm (average 502 ppm) in the early Miocene. The much higher values of pCO2 at the OMT indicate that pCO played an important role in climate dynamics during this time, potentially including the abrupt termination of glaciations.