Senior Constable Bruce Mellor underwent surgery in Palmerston North Hospital late last night to treat lacerations to his face, head and body after the brutal attack north of Taihape around 6.20am.
Mr Mellor stopped a red Mazda 323 hatchback after police received reports about the northbound car driving erratically.
It appeared Mr Mellor got the driver's details and was returning to his car to do a radio check when he was attacked from behind with a machete, Detective Inspector Chris Bensemann said.
As well as receiving deep cuts to his head and nearly losing a finger, he suffered a fractured jaw and eye socket, and had several skull fractures.
"I think the most telling of all the injuries, that provides you an appreciation of what he went through, are the horrific defence injuries," Mr Bensemann told NZPA.
Members of the public stopped shortly afterwards and rang police, but Mr Mellor was also trying to radio for help during the attack.
"It was really quite unexpected, it was a routine stop," Mr Bensemann said.
The constable's eyes were completely closed due to swelling caused by repeated blows to the head.
Two youths, aged 14 and 18, have been charged with assault, unlawfully taking a motor vehicle and attempted arson and police were considering laying further charges.
The 14-year-old was placed in Child Youth and Family custody.
Both will appear in Whanganui District Court tomorrow.
