States of emergency have been issued after thousands were left without power and others evacuated in the wake of ex-cyclone Fehi.
The storm has brought winds of up to 160km/h in some areas, torrential rain and huge tidal surges which have battered coastal land.
Dunedin City Council and the Buller District Council issued states of emergency this afternoon.
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Dunedin mayor Dave mayor Dave Cull said 18mm of rain an hour had fallen on the city and the stormwater system could not cope. Some untreated sewage had flowed out onto the streets.
Buller mayor Garry Howard said low-lying areas and homes have been inundated by the storm surge, with houses now on "residential islands". Some of these had also been flooded with sewage after a pipe failed. Evacuated residents should not return home until advised it was safe. Many were without power.
Westport Airport was closed today and tomorrow due to extensive damage and Buller Health evacuated some palliative and aged residential care patients due to concerns over potential flooding.
Mr Howard said a number of residents would be spending the night elsewhere, as there was still concern about a king tide early tomorrow.
Further south, in Hokitika, Transpower warned people should be prepared for up to two days off the grid, with 6500 properties without power.
Westland District Mayor Bruce Smith said there were powerlines down everywhere, hundreds of trees had fallen and foam from the sea was washing down the town's main street. Mr Smith said he had never seen anything like it.
Greymouth's town centre was cordoned off and sandbags were in place, with the council fearing people could be injured by damaging winds.
State Highway 6 between Hokitika and Haast as well as the stretch of the highway between Westport and Greymouth were closed.
In the Nelson region, about 50 people were evacuated from homes and campsites in low-lying coastal areas including from Collingwood, Mapua, Monaco and Ruby Bay.
The Red Cross had activated its disaster relief and welfare teams in the area.
A failure at the Richmond sewerage pump station has caused sewerage overflows into Monaco, Nelson Haven and Waimea Estuary. Do not swim in these areas.
People were evacuated from their homes in the Taranaki town of Waitara, with the Fire Service worried that if they did not leave they could be stranded when the next king tide hits tonight.
About 1000 homes in the region, and Whanganui, are without power because of either wind or trees bringing down lines.
In Wellington, trees have blown down and wheelie bins became missiles, the Wellington Council said.
A spokesman said bins were blowing around in Newtown and southern suburbs and the council warned motorists and pedestrians to take care.
Trees have come down in many places and the Southern Landfill was closed because of dangerous winds.
Motorists were warned to expect delays on the road and rail lines out of the city due to the weather.
In Wairoa and Manawatu, the Fire Service said fires have been sparked by powerlines coming down in dry areas.
It warned that could happen in other places, even where there has been some rain because trees and grass are so dry.
Air New Zealand said many flights were disrupted across the country. All flights in and out of Queenstown have been cancelled.