Two police officers have been described as "heroes" after they spent an hour submerged in the Manawatu River keeping the head of a truck driver above water.

Manawatu Area Commander, Inspector Sarah Stewart, late this afternoon praised those involved in saving the lives of a man and woman hurt when the truck they were in plunged 50m into the Manawatu Gorge.

The truck was partially submerged following the crash late this morning, but the two occupants, both aged in their 40s, survived and are in stable conditions in Palmerston North Hospital.

That was thanks to a co-ordinated expert response from emergency services, Stewart said.

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"Our officers along with all the emergency services at the scene put their lives at risk today to rescue the man and woman who were in the truck. The flying from the pilot of the Philips Trust Helicopter was incredible and the co-ordinated response from the Palmerston North City Council rescue team, along with fire, ambulance and police services was life-saving."

Senior Constable Ross Andrew and Constable Simon Ashton, both of Palmerston North, were first on the scene and immediately went to the aid of the two people in the truck, she said.

"The female passenger was rescued from the cab first followed by the driver. The officers stayed with the driver in the water for approximately one hour keeping his head above water until the rescue helicopter arrived."

The rescue also involved a paramedic being winched down from the helicopter to the crash site for the driver and passenger.

The scene of the crash. Photo / Supplied
The scene of the crash. Photo / Supplied

Once the injured pair were safe, one of the police officers was winched out while the other was able to climb out of the gorge with the help of ropes and other staff, Stewart said.

Both officers received hospital treatment for hypothermia before being released this evening, she said.

She was "incredibly proud" of both men.

"They are heroes. They went above and beyond the call of duty and put their lives at risk to save the life of the truck driver and his passenger."

Dramatic footage has emerged of the rescue helicopter winching two injured people to safety after a truck plunged into the gorge.

The Palmerston North Rescue Helicopter was buffeted by strong winds and rain as a paramedic helped pluck two people from the gorge.

The footage was uploaded to the police Facebook page this afternoon.

One person was winched from the truck cab while the second was helped from near the truck, the Philips Search and Rescue Trust confirmed this morning.

The truck's occupants - a man and woman aged in their 40s - remain in a stable condition in Palmerston North Hospital. Three of their rescuers were also taken to hospital, but were not admitted.

Motorists who came across the wreckage on the rocky Manawatu River's edge were stunned to see a man standing on top of the crumpled truck's cab this morning.

Two people were winched to safety and are now at Palmerston North Hospital. The extent of their injuries are not yet clear.

Two police officers and a fireman involved in the daring rescue were also transported to hospital after the effects of the chilling water temperature and cold weather.

Hawke's Bay Federated Farmer's president Will Foley was left shaken after seeing the submerged wreck which left a wet Manawatu Gorge Rd - about halfway through the gorge - at around 10.30am.

Thinking it was impossible for anyone to survive, he did a double-take when he saw a person standing on the upturned crumpled chassis.

"All I could see was the chassis submerged upside down in the water," he said.

"Then I saw was a guy in a fluoro jacket standing on the truck.

"It's a long way down. It's at least 50m if not more. I can't believe anyone survived."

Another witness Travis Quin says, "Coming through the gorge, you could see wreckage at the bottom of the gorge. It was pretty mangled."

He stopped to get out and look over the barrier.

"You could just see the cab of the truck. They'd already got a paramedic to abseil down there and the driver was laid on the bonnet of the truck with blankets over him and a paramedic right next to him.

A motorist who came across a truck crash down a steep gorge ravine says he couldn't believe it when he saw someone standing on top of the crumpled wreck. Photo / Will Foley
A motorist who came across a truck crash down a steep gorge ravine says he couldn't believe it when he saw someone standing on top of the crumpled wreck. Photo / Will Foley

"Everyone was quite amazed the driver was still there and they were working on him.

You'd assume from that sort of fall, and looking at the wreckage, that you'd be dead pretty much instantly."

The white B-train truck with two trailers was carrying two red containers with bags of milk powder.

The Fire Service managed to reach the pair and successfully pulled them from their crumpled cab.

Manawatu Fire Service area manager Mitchell Brown said it appeared a police officer had been able to assist the two men from the cab of the truck.

Three ambulances were on scene at 10.44am.

Palmerston North Rescue Helicopter battled rain and strong winds to winch a St John advanced paramedic to the wreckage to help extract the two patients.

One person was winched from on top of the truck cab while the second was helped from near the truck, said the Philips Search and Rescue Trust.

The patients were then met by waiting ambulance crews at the helicopter landing site.

A St John spokesman said a total of five patients had been transported by ambulance to Palmerston North Hospital by 11.58am. These included the two men from the truck, who were in a status three condition, along with the two police officers and fireman.

It is understood no hazardous substances were on board the truck.

The Manawatu Gorge has now re-opened following the dramatic truck crash and rescue.

The New Zealand Transport Agency's acting highway manager Ross I'Anson said the gorge would be under temporary traffic management overnight, with the road near the crash site reduced to one lane and controlled by traffic lights.

"The truck remains in the river. We are waiting for the weather conditions to settle before it is safe to get a crane on to the site to remove the truck. The truck recovery operation will require further road closures and we will inform people via our website, social media channels, and electronic variable messaging signs at Ashhurst and Woodville."

Travellers should plan ahead if travelling through the gorge in the next couple of days, Mr I'Anson said,.

Regularly updated information was available from the agency's traffic and travel information on its website, www.nzta.govt.nz

It's central North Island Twitter feed and Facebook page will also carry up to date information. Updates are also available by calling 0800 44 44 49

When the road is closed for recovery, alternative routes were Saddle Road or the Pahiatua Track, he said.