Towering waves 10m high are crashing off the South Island's east coast.
Niwa says the huge swells have been recorded off Banks Peninsula and are due to hit the North Island later today.
Ferry sailings have been cancelled and coastal roads around the capital are expected to experience surges as the towering seas coincide with tonight's high tide.
"We are getting reports off Banks Peninsula there have been waves of 10m this morning. The waves are well and truly there," said Niwa forecaster Chris Brandolino.
Forecasters were keeping a sharp eye on the enormous waves, most peaking around 6m, as they moved up the country.
Waves of up to 7m are expected in Cook Strait this evening.
Motorists are being warned to take care on coastal roads, especially around high tide at 8:45pm.
But a reprieve is coming. The MetService said the strong winds were expected to drop off today and had lifted a number of warnings.
Warnings were still in force for Clutha, Dunedin, Port Hills and Banks Peninsula.
A gust of 140km/h had been recorded at Le Bons Bay on Banks Peninsula.