Motorists watched an Army truck jump a road barrier, tumble down a 100m drop and disappear into a river yesterday, leaving one soldier dead and two others presumed dead.

The truck, travelling on a narrow, winding road to Queenstown, crashed through the barrier and rolled down into the Kawarau Gorge at 1.30pm.

It landed in a swift, deep section of the Kawarau River and sank immediately.

The three male soldiers were from Burnham Army Camp near Christchurch. They were on a driver training exercise involving at least two Unimog trucks.

They had travelled from Dunedin and were thought to be heading to Queenstown as part of the exercise.

The accident is the fourth in less than six months involving Unimog Army trucks.

Sergeant Steve Ereckson of Cromwell Police said the truck had hit the metal barrier - "probably because of the narrowness of the road and the oncoming traffic".

The truck bounced off and then hit the barrier again.

"The reasons could be from over- correcting from the driver or anything," he said.

The vehicle had then "jumped" over the barrier and dropped off the cliff.

A rescue worker was winched into the gorge to recover the body of one of the soldiers part-way down a drop into the river.

The other two soldiers were still missing last night after a police air, land and jet ski search.

A police dive team will continue the search in the fast-flowing river today. Defence Minister Mark Burton yesterday expressed "deep regret" at the loss of life and injury and said his thoughts were with the families of the soldiers involved in the tragedy.

The accident comes after three others involving Defence Force staff and Unimog trucks last year.

In August, Private Daniel Kairua, 22, from Mangere and Private Sean Dougherty, 29, of Timaru, were killed after the truck they were in tumbled 300m down a steep bank on a Banks Peninsula summit road, near Christchurch.

The men were on a driving exercise involving four Unimogs and the roads on which they were travelling were winding and remote.

A third soldier survived.

An inquiry into the accident had been completed and, while the inquiry would not be made public, a report into the crash might be released, a Defence Force spokeswoman, Major Denise Mackay, said.

In the same month, a Unimog rolled down a bank on to its roof during an exercise inland from Greymouth. No one was hurt.

And in October, three soldiers, two of them women, were injured when a Unimog crashed off the road on the Wairarapa coast.

The three were the only occupants of the truck when it went off the road near Wimbledon in the Cape Turnagain area, southeast of Dannevirke.

Major Mackay said that the Army had 422 Unimog trucks. They were made by Mercedes Benz, but were assembled in New Zealand.

The Defence Force bought the first trucks in 1982. The last of the fleet date from 1986.