A state of emergency has been declared in Nelson as residents have been told to evacuate their homes immediately after the Maitai River breached its banks.
Some 223 homes in Nelson have already been evacuated.
Nelson Mayor Rachel Reese said the river, which runs through the city and out to Tasman Bay, breached its banks as heavy rain continues to sweep across the South Island.
Flood waters are encroaching on the city centre, a video from the Collingwood St bridge shows, while search and rescue teams are now looking for people who need help in the flooded streets and suburbs.
Photos tonight from a Herald photographer in the city show flood waters are just below street level on the Bridge St bridge near central Nelson.
Nelson Tasman Civil Defence is urging anyone who lives near the river from the Nile St Bridge to the Waahi Taakaro Golf Club to evacuate.
"Stay with friends and family if you are able," the advice read. "If you have no safe place to go, head to the Saxton Stadium. Take pets, essential medications and warm clothing."
One elderly couple had to be rescued by Landsar from their residence on the Riverside. Rescue crews had to use a dinghy in order to reach the couple.
People are also advised to avoid bridges and rivers. Some parts of the Nelson region, including the Marsden Valley area, have been without power, Network Tasman said in a notice.
Tasman District Police are advising motorists to limit their travel to essential travel only, as the current severe weather event has closed several roads in the region, with more closures likely.
Reese said there are preparations under way to look after people who need to leave their homes and there will be trained people on hand to offer support.
"If people have no safe place to go, they should head to the Saxton Stadium where they will be able to gain assistance."
NZ Defence Force personnel will be patrolling the evacuated areas overnight to keep an eye on properties.
Wastewater treatment is under pressure and residents are urged to take care around floodwaters and not let children play in it because the water could be contaminated and cause illness.
West Coast-Tasman MP Damien O'Connor is due to arrive in Nelson tomorrow and Emergency Management Minister Kieran McAnulty is expected to be in the region on Friday, 1News reports.
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.
Comment: Update August 18
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