Size‐fractionated phytoplankton standing stocks and potential primary production (PPP) off the west and east coasts of South Island, New Zealand, were evaluated in austral winter and spring 1993. These are the first size‐fractionated primary production data to be reported from the southern Subtropical Convergence (STC) and in oceanic New Zealand waters. Picophytoplankton (< 2 μm) formed > 30% of integrated chlorophyllaand daily PPP in most water types and seasons, except when the 20–200 μm size class dominated in west coast waters in spring and in the STC in winter and spring. In subtropical and STC waters, PPP was 30% higher than at similar latitudes in the North Atlantic Ocean. Interpretation of whether or not there was accumulation of chlorophyllain the various water types and seasons depends on microzooplankton grazing, because mesozooplankton grazing and the proportion of chlorophyllous particulate matter sinking out of the upper water column was low. In the STC in spring, phytoplankton growth was slowing down (potential doubling times were at most 1.01 per day at 10 m and PPP per unit biomass was lower than in water masses to the north and south), probably because of decreasing concentrations of dissolved reactive silica. A sedimentation event of diatoms is postulated to have occurred in the STC soon after our spring sampling dates. The balance between algal growth and loss factors in the south‐west Pacific depends on the unique combination of physical and chemical environmental conditions experienced there. The implications of the distribution of phytoplankton biomass and primary production by size‐fraction for carbon export is discussed.