This study is motivated by the potential improvement in water supply reliability and better forecasts of extreme rainfall and floods linked to Atmospheric rivers (ARs) in New Zealand. Results indicate that ARs generally dominate monthly rain amounts and wet and dry conditions for the western side of mountainous regions and the north part of the country. Precursor soil moisture conditions and their relationship to runoff are also evaluated. In these regions, ARs are more likely to replenish soil moisture or lead to flash floods in warm-season months but generate severe floods in cool-season months due to low soil moisture deficit. This relationship is further confirmed by the seasonality of the monthly maximum streamflow of the Whanganui River (a major river in the North Island of New Zealand). Additionally, ARs generally lead to high streamflow all year round and are responsible for most top 10% streamflow.