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New Zealand climate during the early to middle Miocene was subtropical to warm-temperate, in contrast to the predominance of cool-temperate climates in New Zealand today. Both modern and Miocene environmental settings of New Zealand are strongly moderated by the surrounding ocean. Seasonal moisture deficits occur today in rain shadow regions in New Zealand, but the potential and nature of seasonal moisture deficits under globally warmer conditions is uncertain. Here, we reconstruct seasonal moisture balance (precipitation - potential evapotranspiration) using seasonal temperature and mean annual precipitation estimates derived from early to middle Miocene (23-11 Ma) fossil leaf assemblages, combined with a Penman-Monteith model for potential evapotranspiration. The model incorporates uncertainty in temperature, precipitation, vapor pressure deficit, radiation and wind speed. Our results suggest that three out of nine sites investigated have a very high potential for summer moisture deficits (lowest monthly moisture balance < -90 +/- 40 mm month(-1)), without considering potential differences in infra-annual rainfall. These three sites all have the potential of an annual negative moisture balance (< -230 +/- 460 mm year(-1)). The highest negative moisture balance in New Zealand today is similar to -400 mm year(-1). Additionally, the three sites with the highest potential for moisture deficits all are from the Manuherikia Group, of which the depositional period may partially overlap with the Middle Miocene Climatic Optimum. Other sites had moderate (lowest monthly moisture balance < -40 +/- 50 mm month(-1)) to low (lowest monthly moisture balance < 45 +/- 60 mm month(-1)) potential for moisture deficits. Moisture deficit potential appeared to be highly dependent on the reconstructed mean annual precipitation. The trend from the early/middle Miocene to middle/late Miocene boundary of decreasing potential for moisture deficits agrees with a global cooling trend and regional increase in precipitation. Our study highlights the importance of potential evapotranspiration in globally warmer conditions, as seasonal moisture deficits may occur even in environments that are considered strongly ocean-moderated, such as New Zealand.