Two young children critically injured in a Waikato two-car collision this morning were not in approved children's car seats, police say.
Two SUVs - a Kia Cerato and a Toyota Hilux towing a trailer - collided on State Highway 27 at Patetonga, north of Morrinsville, about 5.40am.
Acting District RoadPolicing Manager, Senior Sergeant Jeff Penno said the trailer being towed by one vehicle was carrying another vehicle, and ended up on its side in the accident.
The family of four inside were all hospitalised.
"The two children ... were not restrained in approved car seats and have suffered critical, and serious to critical injuries," Mr Penno said.
Westpac Waikato Air Ambulance pilot Dan Harcourt said a five-year-old boy suffered a serious left arm injury and possible internal injuries was flown with his father, who suffered minor cuts and abrasions, to Starship Hospital.
His older brother suffered a head injury, and was flown with his mother, also with cuts and abrasions, to Starship by the Auckland Westpac Helicopter.
Mr Penno said the female driver of the Kia suffered moderate injuries, and was taken to Waikato Hospital in Hamilton, while her male passenger suffered minor injuries in the crash.
"The Waikato Serious Crash Unit is working to establish the cause of the collision and initial indications are the driver of the Hilux may have left the road while cornering and over-corrected, crossing into the path of the Kia.
"One thing that is readily apparent however is the young children were not in approved car seats. This is inexcusable on the part of drivers who should take every precaution possible to safeguard that most precious of cargoes- our children."
Police said SH27 is expected to be remain closed about 12pm to allow crash investigators to complete their preliminary scene examination.
