Civil Defence has evacuated all residents in the area surrounding the Clarence River as a surge of water up to 15m high rushes downstream after breaking through the earthquake debris that had slipped into the river earlier this afternoon.
They were being taken to Blenheim in vehicles, a Civil Defence spokeman said.
"The road is closed, but we think we can get them out over land."
The spokesman said Civil Defence had "no concerns" that anyone was in danger of the river.
The evacuees were from homes nearby the slip and much of the area between the slip and the river mouth was farmland.
It was likely residents at the eight to 10 farms self evacuated and Civil Defence personnel would check in with them in the morning, he said.
Homes near the river mouth were evacuated this afternoon.
A group of rafters, who were rescued earlier today, and around 20 Marlborough tourists, were also driven to Blenheim.
A dozen kayakers reported missing on the Clarence River have been found safe.
The group, from the National Outdoor Leadership School, were evacuated by chopper today, the Civil Defence spokesman said.
Authorities located 16 rafters out on the flooded river as a wall of water rushed downstream at speeds of 1000 m3/s to 2000 m3/s from the dammed-up section near the Dart Stream 50km to the sea.
A one-in-10 year flood event would usually flow at 1600 m3/s and a one-in-100 year event at 2500 m3/s.
Police contacted the rafters via satellite phone and pinpointed their location when the call was made.
A helicopter from Nelson hovering over the river spotted the breach.
Civil Defence ground teams were en route to the river.
Water had built up behind the slip, about 10 to 12 kms upriver from the mouth.
The water broke through the dammed-up debris around 4.20pm.
Civil Defence is urging nearby residents to get to higher ground immediately.
Emergency workers began evacuating nearby residents around 3.30pm, before the slip breached as a precautionary measure.
Earlier today, Fish and Game urged anglers to steer clear of the Canterbury river, due to a slip blocking the river in the vicinity of the Dart Stream, with a lake forming behind the blockage.
Gretchen Dalzell, who guided tours on the Clarence River last season, told Fairfax there would have been four to five guides experienced on grade two rivers like Clarence on today's trip.
"It's a commercial trip. They must have left around Thursday or Friday last week.
"At this time of year we would do five-day trips. We start at Jacks Pass, about a 40-minute drive from Hanmer, and get into the Clarence River from there.
"We would camp on the way down. We're quite aware it's a very steep valley, and there are lots of areas you might camp where, in an earthquake, it would be quite treacherous. There's the potential for lots of rockfall. And that's a real worry for that group."
She said the guides would have had emergency management training and known how to use a satellite phone and administer first aid.
"But, I mean, how much can you do to prepare for that?"