Phytoplankton assemblages and their physicochemical environment were investigated during 1997-98 at a marine long-term coastal monitoring station off Sydney, Australia, and compared to those previously seen at this location. Phytoplankton blooms (significant population increases) coincided with episodic slope water intrusions (upwelling/uplifting) lasting 2-22 days and occurring from September to February. These blooms appeared to occur in response to slope water intrusions irrespective of this location\'s proximity to other major nutrient sources. The hydrological forcing variables of bottom- and surface-water nutrients and temperature, (including time-lagged data), were identified using Principal Component Analysis as those variables that explained 60% of the variability of the total phytoplankton biomass throughout the year. Phytoplankton blooms of similar frequency and magnitude to those seen in this study have been previously recorded. However, in contrast to earlier work, where a variety of taxa dominated throughout the year, the small diatom Thalassiosira partheneia generally dominated blooms in this study. In addition, presence/absence data for the heterotrophic dinoflagellate, Noctiluca scintillans, indicated a higher frequency of occurrence for this species than previously documented. N. scintillans was observed in 61% of samples collected throughout the year, being absent from only a few samples in the autumn and winter months. It is hypothesised that the shift in dominance from previously recorded bloom species to Thalassiosira could be a contributing factor to the increase in N. scintillans (a favoured food source by N. scintillans ) in NSW coastal waters. The reason for the recent dominance of these particular phytoplankters is unclear but may be related to physicochemical conditions such as a decrease in phosphate and oxidised nitrogen concentrations and warmer water temperatures experienced during our sampling period compared to previous years. This includes what appears to be the first record of the potentially toxic diatom Pseudo-nitzschia australis in Australian waters and the first record of the harmless diatom Pleurosigma chilensis since its "type" description from the coastal waters of Southern Chile, 1941.