A new study prepared by Earth Sciences New Zealand for Northland Regional Council examines how different weather systems produce different river flood responses across Northland.
Using long-term river flow records, the study developed a regional Storm–Flood Typology based on three characteristic responses. Type 1 floods rise and fall quickly and are generally short-lived. Type 2 floods rise rapidly to a pronounced peak before receding more gradually. Type 3 floods develop more slowly and can keep river levels elevated for several days.
Type 2 was the most common response, accounting for 43.1% of monitored sites, followed by Type 3 at 32.3% and Type 1 at 24.6%.
The framework links localised convective storms with rapid Type 1 flooding, extra-tropical cyclones and frontal systems with Type 2 regional flooding, and cyclones enhanced by persistent atmospheric-river moisture with widespread, longer-lasting Type 3 flooding. Widespread hydrological events occurred most frequently during winter, accounting for 40% of the events identified in the study.
The Storm–Flood Typology provides NRC with a consistent way to interpret how developing weather systems may translate into river responses. It will help distinguish between rapid, localised flooding and widespread, prolonged flooding, supporting operational preparedness, flood-event interpretation, and post-event review across Northland.
The report was prepared by Earth Sciences New Zealand for Northland Regional Council, with funding from Envirolink.
Published: 29 Sep 2026