Cyclone Gita is pounding New Zealand with torrential rain, damaging winds and large waves.

Tens of thousands of people have lost power, a number of major roads are impassable and flights to and from Wellington International Airport have been suspended until the cyclone has cleared.

There are also reports of people being trapped on their roofs by flood waters and one person being swept away, according to the New Zealand Herald.

Seven provinces - Christchurch, Buller District, Grey District, Selwyn, Westland, Tasman and Taranaki - have declared states of emergency.

Waves as high as 6.7 metres have been pounding the coast and a gust of 130 kilometres per hour (kmph) was reported in Hawera.

A staggering 256 millimetres of rain fell in the Kaikoura Ranges in just 18 hours, and of that, 53.6mm of that fell in one hour, making it the wettest hour ever recorded in New Zealand.

The cyclone developed two weeks ago in the southern Pacific Ocean near the waters of Fiji.

It then rapidly strengthened as it grazed Niue and headed towards the islands of Tonga, which was badly hit. The entirety of the main island of Tongatapu lost power and the storm destroyed numerous homes, churches and even the nation's historic Parliament House.

Gita then skirted the south of New Caledonia, but caused no major damage, before curving to the south and running towards the land of the long white cloud.

The storm is expected to take about 6 hours to cross the South Island, with the centre of Gita clearing the east coast around 15GMT (3am local time).

Even as the storm clears, more rain is forecast on Wednesday, exacerbating the flooding problems and hampering the clear up efforts.