The North Island could be battered by up to two tropical cyclones over the coming summer.
Forecasters across the Southwest Pacific show many countries in the region have a higher-than-usual risk of category 3 or higher cyclones from November to April.
Oceanic and atmospheric forecasts show La Nina conditions will likely develop, which consist of below-average sea temperatures in the Pacific.
Niwa scientist Chris Brandolino said this would bump up the likelihood of cyclones.
"For the whole season, my expectation for the upper half of New Zealand is for an above-average cyclone risk."