"Unrestrained, the baby has been thrown into the rear interior of the car and suffered critical injuries," said Waikato road policing manager Freda Grace.
A person known to the family, who are believed to be from Taumarunui, was the first on the scene and is understood to have taken the infant, who is less than a year old, to Te Kuiti Hospital.
The baby was airlifted to Starship children's hospital in critical condition.
The baby's 3-year-old sibling and their mother were taken to Waikato Hospital by ambulance with minor to moderate injuries while three other adults were treated at the scene.
"Their injuries are insignificant next to the child that was unrestrained," said Ms Grace.
Crash investigators are working on establishing what, if any part, two bald tyres on the front of the car played in the driver losing control on the downhill stretch of road.
Ms Grace said it appeared the car was travelling faster than the advised limit and she described the state of the front tyres as "not in the best condition".
She said this was an example where the driver had not taken all the necessary precautions.
"We talk about road policing being everybody's responsibility and that actually starts with the driver of the motor vehicle in the first place."
Caroline Perry, a spokeswoman for road safety charity Brake, said parents had a responsibility to their children to ensure they are protected in vehicles and in proper restraints.
"Allowing your child to ride without one is putting them at huge risk and can have devastating consequences," she said. "This is an absolutely tragic incident. Unfortunately it does demonstrate that children can suffer horrific injuries if they are not properly restrained in a vehicle."
The crash yesterday at 8.30am was part of a horror run on the country's roads.
25-year-old Hina Hamahona died after crashing into a ditch at a Hawkes Bay blackspot. It's understood he made a final phone call to his father while trapped in the wreckage.
Last night a woman died in a car crash on State Highway 3 at Ohaupo, near Hamilton.
Two cars travelling in opposite directions collided about 5.50pm, police said.
Another person was airlifted to Waikato Hospital with moderate injuries.
Shannon Kiriau. Photo / Supplied by Facebook
On Sunday, brother and sister Shannon and Danielle Kiriau died in a high-speed accident in Dunedin in which none of the back-seat passengers were wearing restraints.
Police say the green-stickered car was travelling at 180km/h before the driver lost control while negotiating a slight right hand bend on State Highway 1 near Green Island and spun several times before striking a light pole and a tree.
