Motorcyclist clings to tree in Waimakariri River

The rider was able to cling to a semi-submerged tree for around 40 minutes. Photo / Adonia Wilson

A truck driver thought he'd killed a motorcyclist when he lost control on a Canterbury bridge this morning, sending the rider plummeting into the freezing river below.

The rider, a 47-year old from Rangiora, clung on to a semi-submerged tree for around 40 minutes before a rescue helicopter winched him to safety.

Only his thick riding leathers saved him from succumbing to the chilly waters, police say.

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The truck driver, a Scot who remained at the scene and did not wish to be named, was shaken up the accident.

He said he was crossing the narrow Old Waimakariri Bridge about 7.20am when he looked down to the choppy, flooded muddy waters below for a moment, before his right rear wheel clipped the concrete kerb and he lost control.

He was sent into a skid, broadsiding across both lanes, in front of the new Harley Davidson bike.

"The bike had pretty much stopped and he went over the railing," said the driver, who police say may yet face charges.

"I jumped out of the truck on the passenger's side and tried to get a rope to him.

"He was hanging on to a tree (about 30m from bridge).

"Me and some others got three straps to him but he says he couldn't hang on. He was just too cold. He had no feeling in his arms. Freezing, man."

A close call for the driver of this motorcycle who ended up clinging to a tree in the flooded Waimakariri River this morning. Photo / APNZ

The rider was suffering from hypothermia, said Inspector Malcolm Johnston at the scene, but otherwise was uninjured.

He was saved by wearing full leathers, the senior officer thought.

The driver agreed.

"I'm just so glad he's OK."

Constable Lisa Walker of Kaiapoi was the first police officer on the scene, after abandoning her breakfast.

She said the driver had thrown a strop but the rider was just "clinging to the tree for dear life".

After telling the rider a helicopter was on the way and to hang in there, her main job was to stop the driver from jumping in the freezing waters himself.

"He was going to (jump in)... and then we would've had two people to rescue."

She also thought the leathers saved the rider.

"He wouldn't have lasted long without them."

A large crane has now been brought in to removed the truck and clear the bridge.

Mr Johnston said the rider was lucky the river was in flood, otherwise he would have had a 15-metre fall onto gravel.

Police communications alerted the Westpac Rescue Helicopter and Coastguard Ashley-Waimakariri who were on the scene shortly before 8am.

The Westpac Rescue Helicopter lowered a paramedic and winched him to safety.

The helicopter landed on the bridge where the motorcyclist was transferred to an ambulance which took him to Christchurch hospital. He was described as being very cold but had suffered only minor injuries.

"The man is extremely lucky to be alive," says sergeant Mike Brooklands.

"If he had lost his grip on the tree it's unlikely he would have survived in those conditions.

"The rescue personnel have done an incredible job to pull him out of the situation."

The driver of the truck was assessed by ambulance staff at the scene but was not injured.

Police also praised the efforts of the emergency services that responded to the incident including the rescue helicopter, ambulance, coastguard and police personnel that were on the scene.

Police enquiries into the crash continue.

The bridge is expected to remain closed for several hours and traffic is being diverted.