Six teenagers and a teacher have been killed in a flash flood on a river in New Zealand. The river level had risen sharply in half an hour, sweeping some of them downstream through a gorge.

A group of 12 students and teachers were on an outdoor leadership course when they were hit by floodwaters late yesterday in the Mangetepopo Gorge, in an isolated area near the centre of New Zealand's North Island.

Two bodies were carried 2.5km down the Mangetepopo, while five other bodies were found a short distance down river from where the group first got into trouble.

Four students and an instructor survived the flooding. One of them was taken to hospital with back injuries.

Grant Davidson, chief executive of the Sir Edmund Hillary Outdoor Pursuits Centre which organised the trip, said there had been no warning of the heavy rain and when the students entered the gorge the river was at a low level.

All were wearing wetsuits, lifejackets and helmets and were led by an experienced instructor, but the river rose 36-fold within 30 minutes.

'The Mangetepopo Gorge is steep and narrow-sided in parts and littered with large boulders, which would have been unforgiving to anyone being swept through the water,' said senior police constable Barry Shepherd.