Noctiluca scintillans ,a large heterotrophic dinoflagellate, has been present in New South Wales coastal waters since 1860. Noctiluca was a minor component of the phytoplankton up until the last decade, but since then, Noctiluca blooms appear to have increased in frequency and intensity, with the majority of blooms recorded in recent years. There has been public concern that this increase in Noctiluca blooms may be a result of chronic sewage discharge from three deepwater ocean outfalls in the coastal waters surrounding Sydney. Such concern is consistent with the debate that Noctiluca maybe a coastal eutrophication indicator species. Over a one year period (1997-1998) at a long-term monitoring station off Sydney, Noctiluca was present year round with peak abundances coinciding with episodic slope water intrusions (and subsequent upwelling) during spring and summer. Extensive red tides of Noctiluca often succeeded diatom blooms (specifically Thalassiosira spp.) which were initiated by these intrusions.  80-90% of Noctiluca cells contained food particles and there was a positive linear relationship between abundance and the proportion of cells containing food. Prevalent food items within the vacuoles of Noctiluca were diatoms. Water samples collected north of Sydney also showed that highest numbers of Noctiluca occurred in areas predisposed to upwelling. Upwelling is likely to be the mechanism that promotes population growth of Noctiluca along the coast of N.S.W. Despite these findings, it is still difficult to disregard the direct or indirect effects of anthropogenic nutrients on the growth of Noctiluca. A long term assessment of physico-chemical dynamics in the water column off Sydney showed that there was no real change in phytoplankton biomass nor in uplifting/upwelling frequency in the last decade, yet the recent year round presence of Noctiluca is unprecedented for this region. A shift in the dominant diatom genera to Thalassiosira (believed to be the optimal food source of Noctiluca) concomitant with a rise in temperature and reduction in nitrate and phosphate levels may be the cause for the increase in Noctiluca numbers during this sampling year. The presence of a strong ENSO signal in 1997-1998 was shown in companion studies to dominate physico-chemical conditions, and thereby mask any anthropogenic effects. Future investigations need to incorporate this influence to resolve and partition climatic and anthropogenic signals.