Two other females, who were sitting in the back, also received serious injuries.
A 27-year-old man with minor injuries was treated at the scene.
The other car was driven by a 28-year-old man from Putaruru. He was in Rotorua Hospital last night with serious injuries.
His two passengers, also from Putaruru, were taken to Taupo Hospital with minor injuries.
The flames are believed to have been caused by the hot engine of one of the cars setting grass on fire.
"Once again the fast actions of people saved lives," Mr Dodds said. "One of the cars caught alight and was quickly put out by a father and son travelling home."
Three helicopters and four ambulances took most of the survivors to Rotorua Hospital with injuries ranging from moderate to serious.
Senior Sergeant Fane Troy said a scene examination showed one of the cars crossed the centre line and collided with the other.
He said crashes weren't common in the area because the road is so straight.
The serious crash unit was investigating, but speed and alcohol weren't thought to be factors.