A truck driver is carried to a waiting helicopter after his vehicle crashed into the Rangitikei River.

A truck driver ended up partially submerged after his vehicle left State Highway 1 and plunged into the Manawatu's Rangitikei River.

The man was helped from the wreckage by fellow truck drivers after the accident, early on Tuesday morning.

The truck carrying potato chips left State Highway 1 about one kilometre north of Mangaweka, and crashed into the river at 5am on Tuesday.

The truck flipped and was stuck wheels-up in the fast-flowing river, police said.

Other trucker drivers on the road stopped and made their way down to the river, smashing a window in the cab of the truck to free the driver.

"The driver once assisted from the truck, though wet and cold, was able to walk up to emergency services who had arrived," police said.

A Palmerston North based Rescue Helicopter spokesman said the truck driver was partially submerged before he was freed.

Due to low cloud at the crash site, the driver was taken a short distance by ambulance before he was flown to Palmerston North Hospital.

Mangaweka firefighters helped take the injured man to the helicopter.

The Auckland-based driver, in his 50s, was in a stable condition on Tuesday morning.

Police will be investigating the crash.