In the early 1998 summer, sporadic mortalities of fish and marine fauna occurred around the Wairarapa coast, central east coast of New Zealand. Over the period from January to early February 1998, more than 200 people were reported to have suffered from respiratory illness apparently originating from the build-up of the newly described Gymnodinium brevisulcatum sp. nov. in a nearshore bloom. By mid-February 1998, further human respiratory syndromes were reported in Hawke Bay to the north of the Wairarapa coast. Between mid-February and March 1998, an almost monospecific bloom (max. cell concentration recorded >33 million cells per litre) of the same new species occurred in Wellington Harbour, which is located to the southwest of the Wairarapa coast. This unprecedented bloom in the harbour killed a large number of fish, marine fauna, and seaweeds. Similar human respiratory distress, previously experienced along the Wairarapa coast, was also reported around Wellington. Over the period from January to March 1998 large scale changes in oceanographic conditions had been observed on the east coast of New Zealand. Results from a research voyage off the North Island east coast have shown that warmer subtropical water was found further to the south than usual, associated with a strong southward current. This is consistent with observations of warmer than usual satellite sea surface temperature during the period mid-January to mid-February 1998. Examination of seven across-shelf transects stretching around the entire east coast of North Island showed the presence of G. brevisulcatum sp. nov. in the warm, southwards-directed, offshore, East Auckland Current and East Cape Current. In February 1998, highest cell concentrations of G. brevisulcatum, however, were found in the southern section of relatively cool, less saline, northwards-directed, nearshore, Wairarapa counter current. The widespread occurrence of this toxic G. brevisulcatum during the 1998 summer seems to be related to a combination of downstream effect of weather changes associated with the El Nino pattern, and the dynamic interplay of two coastal current systems.