Earth Sciences New Zealand has declared El Nino conditions, warning impacts on the country's weather are expected to intensify through the coming months.
The agency said there is an 80 per cent likelihood the event will reach or exceed strong intensity during the July-September period, with effects peaking over the 2026-27 summer.
Principal scientist Chris Brandolino said conditions in the Pacific Ocean had met ESNZ's threshold, with the ocean and atmosphere now "coupled" – with higher-than-normal sea surface temperatures in the eastern Pacific and weaker trade winds that would normally drive rainfall towards NZ.
Rainfall is likely to be near or below normal for the north and east of the South Island and the east of the North Island, with below-normal rainfall most likely for the rest of the North Island.
The west of the South Island is likely to see above-normal rainfall.
Air temperatures are equally likely to be near or above average nationwide, though cold snaps and frosts are expected.
Increasingly windy and variable conditions are anticipated later in winter and into spring as El Nino signals strengthen.
Two lower-probability risks remain under monitoring: sudden stratospheric warming events, which have become more frequent in recent years, and the remote possibility of an out-of-season tropical cyclone, historically associated only with El Nino years.