Roads were closed and warnings were given about possible contamination of drinking water as 320mm of rain - about two months' worth - fell in 24 hours.
Nelson Mayor Aldo Miccio said there has been more than 200 slips in the region.
"Any amount of rain now will cause slips ... and we expect more heavy rain today," Mr Miccio told Firstline.
A slip had been found under the Maitai pipeline, which supplies water to all of Nelson, and authorities will today attempt to shore up the pipe. Residents are advised to conserve water.
Mr Miccio said the damage could run into the millions of dollars.
Weather warnings lifted
The deluge swept north yesterday to soak Auckland, Bay of Plenty and Northland, keeping emergency services busy as flood waters washed over roads and bridges.
MetService has now lifted all its severe weather warnings as the front which drenched the country has now cleared the North Island.
Civil Defence said the rainfall in Nelson was "well in excess" of a one-in-100 year event and in the past 48 hours the Fire Service has attended 170 callouts.
One search and rescue team was on the scene within seconds when a 77-year-old man was swept from his house in Nelson.
The man was sitting in an armchair in his Pohara home when he was swept out by floodwaters and into a paddock in front of the house.
Search and rescue workers were nearby and got to the man quickly. He was frightened, but not injured.
Tasman police district commander Superintendent Gary Knowles "countless" slips had caused a great deal of trouble and damage to property around the region.
"Police and other emergency services have been working around the clock to manage the issues arising," he said.
"We have some serious as a result of landslides and the resulting damage to property."
Mr Knowles said evacuations continued all day yesterday, but one group remained cut off by slips.
Army staff had been unable to reach them.
"However they have had some assistance from the local community and police do not hold immediate concerns for their safety."
He said most of those evacuated had been returned to their homes.
"However some have been deluged with mud and others are still too close to the danger zone to return. Many residents have also self-evacuated and gone to stay with friends and family."
Like a "tsunami" has gone through
Holidaymaker Paul Altabas and his wife were trapped at their rented bach in Tata Beach, in Golden Bay, as flood waters and debris blocked roads.
Mr Altabas said the debris had been creating dams on land higher than their rental house, near the beach.
As the rain got heavier, the dams began to burst, flushing the debris into lower-lying areas.
"It's spread everywhere. And the floodwater has washed away a few vehicles we've been told. What we're not seeing in the photos on the internet is the houses that have been swept away.
"People with caravans and shipping containers with their possessions in them - they've been washed away too.
"There's no traffic coming through. You can't get to us. It looks like there's been a tsunami come through. There's a large amount of wood and sand, stones and rocks everywhere, mixed in with people's belongings."
Bob Gordon had to hurriedly grab what valuables he could as walls of mud poured down the hill filling his Nelson home on Wednesday.
One of dozens forced to leave their homes after severe flooding, Mr Gordon is now relying on a friend to put him up in an apartment.
"There was a bit of mud coming down the drive, and mud about my height coming down the side of the house. I realised it was serious then.
"My back door was creaking and groaning and ooze was starting to come in under the door and across the floor. So we grabbed everything we could ... and we could see more mud coming down [the hill]."
He had about an hour to save what he could while there were 'thumps and bangs coming down the hillside".
"If the house survives, it won't be habitable for about three months. But I suspect it is off its foundations now."
Errol Guthardt's property of the past 56 years resembled a lake yesterday, with water filling his basement.
"I never felt unsafe, but they keep telling me I should [evacuate the house]. "They're saying the hill above us is suspect. It's not good for a bloke with a crook ticker. The blood pressure's up pretty high."
Last night Weatherwatch analyst Philip Duncan said the rain in Nelson had eased.
"Drizzle is falling in some areas. Mostly dry weather should move in overnight, but heavy showers are possible tomorrow."
What about Christmas?
Christmas Day is likely to be dry and fairly sunny with average temperatures for most main centres, says weatherwatch.co.nz.
Forecasters say a large high will be centred to the east of the South Island and the chance of showers in Auckland, Coromandel and Northland will be lower than 30 per cent.
The only possible problem is another rain-carrying La Nina system from the Coral Sea and sub-tropics.
But weatherwatch says it is more likely to reach New Zealand between Boxing Day and New Year.
