New Zealand floods: hundreds evacuate as 'atmospheric river' brings deluge

A third straight day of rain has brought flooding, landslips and road closure to both the North and South islands

Torrential rain has slammed the west and north of New Zealand's South Island for a third straight day, forcing hundreds to evacuate their homes and triggering road and school closures and land slips.

Coming top of weeks of damp weather, Thursday's rainstorms are worsening conditions in New Zealand's already sodden landscape. Experts have attributed the unseasonably wet weather to a narrow stream of water vapour, or an "atmospheric river", sitting above the country.

Residents in the northern part of North Island found themselves isolated after landslides, fallen trees and flood waters blocked highway access. The stormy weather also forced airlines to cancel flights and businesses to shutter.

The storm sunk at least one sailboat near Auckland and caused a home to slip down into a gully in the town of Tāhunanui.

Weather forecaster Metservice's data showed part of the north of the South Island had received well over 300mm in the past 24 hours. It has heavy rain warnings in place for parts of the west of the South Island and in the north of the North Island.

Metservice data showed Nelson city on the South Island had received 106mm of rain since midday on Tuesday - well above its average rainfall for the whole of August of 80mm.

New Zealand's emergency management minister, Kieran McAnulty, visited Nelson and pledged an initial contribution of $200,000 to the mayoral relief fund to help the communities affected.

On New Zealand's North Island, the country's largest city, Auckland, is under a heavy rain and winds alert, with minimal disruption reported so far.

Authorities said more than 230 homes in Nelson, a city with a population of more than 50,000, have already been evacuated, with many public facilities and roads closed.

A statement on Nelson council's website warned continuing rain could mean more land slips, flooding and evacuations.Nelson's mayor, Rachel Reese, told television show AM that while the city had made it through the night without any major incidents, infrastructure was under pressure.

"We are dealing with a lot of wastewater overflows," she said.

On the west coast of the island Buller district council said in a statement people from 160 homes evacuated over the last day were able to return to their residences to assess damage. But it warned further rain was expected and it was possible that they would have to evacuate again.

"Right across the district I believe we got away relatively unscathed," Buller's mayor, Jamie Cleine, told a news conference streamed online.