New Zealand kauri (Agathis australis) (D.Don) Lindl. is a large and long-lived tree species endemic to the species-rich forests of the north of the North Island. Agathis australis are culturally and ecologically significant, but little is known about their ecophysiology. In particular, environmental drivers of fluxes of carbon and water for A. australis trees have not been quantified. We measured leaf gas exchange to explore the effect of leaf age, tree size, foliar nitrogen concentration, photosynthetically active radiation (PAR) and vapour pressure deficit (D) on assimilation rates (A) and stomatal conductance (g(s)). We also measured carbon isotope discrimination of leaves and applied an optimal stomatal behaviour model. Both g(s) and A were highest for year one leaves (130mmolm(-2)s(-1) and 5molm(-2)s(-1), respectively) then declined with leaf age to < 80mmolm(-2)s(-1) and < 3molm(-2)s(-1), respectively, in 4-5-year-old leaves. Instantaneous water use efficiency (A/g(s)) was highly variable, but there was no leaf age-related pattern. Our diurnal results indicate that A. australis g(s) peaks early in the day (before 0900 h at 250mmolm(-2)s(-1)) and A is comparatively low, remaining below 9molm(-2)s(-1) throughout the day. Overall, water use efficiency is low based on intrinsic water use efficiency and the stomatal model. Isotopic analysis indicated moderate water use efficiency over the life of leaves compared to other temperate conifers. This information is valuable for modelling carbon and water fluxes of A. australis and for improving our understanding of the threat of summer droughts to these forest giants.