Police said they were contacted by the 56-year-old from Christchurch about 10.30am. He was rescued by a Coastguard rescue vessel about an hour later.
Unfortunately, his tent and car were unable to be retrieved, police said.
The heavy rain had eased this afternoon, but a severe weather warning was in place as a burst of heavy rain was expected in Westland, south of Otira, tomorrow morning.
It was possible the downpour would bring between 90mm to 120mm of rain.
The rain was due to active front, following a strong moist northerly flow, that was moving east across much of the country, MetService said.
A severe weather watch was in place for the rest of Westland and Buller, but there was potential this would be upgraded to a warning, MetService said.
A number of slips closed SH6 at the lower Buller Gorge, between Inangahua and Westport earlier today, trapping shaken motorists.
The New Zealand Transport Agency's regional performance manager Pete Connors said contractors worked hard throughout the day to get the highway re-opened.
They cleared 2000 cubic metres of trees, rock and mud which came down at the biggest slip at Summit Creek, he said.
Vehicles and their occupants that became caught in the slip this morning were freed about 12.30pm.
"The occupants were shaken but uninjured by the incident, being assisted from the area earlier in the day by local police," said Mr Connors.
This afternoon, Buller Civil Defence said it had deactivated its emergency operation centre, as the weather had subsided and surface flooding had dropped.
However, the situation would be monitored over the next 24 hours and updates would be provided if required.
In the meantime, residents were encouraged to prepare themselves as much as possible for more heavy rainfall tomorrow.
Motorists were also encouraged to stay off the roads, particularly those areas with surface flooding, as vehicles driving through surface water were causing waves to hit houses, meaning further damage.
Buller Mayor Garry Howard said residents needed to check their properties and check on their neighbours.
Earlier today, he said the surface flooding in Westport was the worst he had seen since 1998, and the town was "moving towards a major event".
Westport firefighters spent most of the morning pumping floodwater which threatened a number of properties, while all four schools in the area were closed due to heavy rains.
Westport Volunteer Fire Brigade acting deputy fire chief Trevor O'Dea said surface flooding in the town this morning was "major" and there was little emergency services could do to mop up flooded properties.
"It's not draining away. There's not really much we can do because there's nowhere to pump it," Mr O'Dea said.
Mr O'Dea said the brigade had attended 15 call-outs by mid-morning due to surface flooding, leaking roofs, and to help people stem the tide of flooding on to property.
Severe weather also battered Nelson this morning, and police closed roads due to strong winds and heavy rain.
Atawhai Drive, in the vicinity of Founders Heritage Park in Nelson, was closed due to the severe weather.
In the capital, strong winds blew three large shipping containers into Wellington Harbour today.
Two of the three bright orange containers were tethered to Clyde Quay Marina until winds died down and they were able to be retrieved from the water.
KiwiRail said a number of Interislander services would be delayed due to the severe weather today.
It was contacting affected customers and advising them of revised sailing times to ensure any delays to travel were minimised.
- Additional reporting: Westport News and Greymouth Star
