The woman driving the Volkswagen suffered serious injuries and was airlifted to Waikato Hospital where she was reported as being in a serious but stable condition overnight.
The women were travelling east on State Highway 25 near the intersection of Pipiroa Road about 7.45am when the crash occurred.
Mr McDonald said the crash was still being investigated by the Waikato Serious Crash Unit to determine the circumstances of what occurred.
"All we can say at this stage are initial indications are the west-bound Volkswagen car appears to have crossed the centre-line and collided with the Mazda in the east-bound lane.
"The female driver of the Volkswagen suffered serious injuries and was airlifted to Waikato Hospital where she was reported as being in a serious but stable condition overnight."
Mr McDonald said neither excessive speed, alcohol, or the not wearing of seatbelts appeared to be factors in the crash.
Road toll spikes as more lives lost on roads
The national road toll has jumped 25 per cent higher than at the same time last year after six deaths on the road yesterday.
Forty three people have died this year on New Zealand's roads, 11 people more than at this time last year.
The rise in the number of lives lost on the roads comes after six people died in three fatal crashes yesterday.
Three people died and one was critically injured after a car and a logging truck collided on State Highway 1 north of Tokoroa about 3.50pm.
All four, believed to be of Asian descent, were travelling in the car.
It is understood the male front seat passenger, who was airlifted to hospital, was the only person wearing a seatbelt at the time of the collision.
The car was believed to have spun out of control before smashing into the truck.
Police said the car crossed the centre line but the cause was yet to be established.
The female driver of the car and two rear-seat passengers, one male and one female, died at the scene. Police confirmed both in the rear were not wearing seatbelts.
The truck driver was shaken but not injured.
Police confirmed the car's passengers were foreign nationals but said no further details, including their nationality, would be released until next of kin had been notified.
Barely two hours after the triple fatality, another person died in a crash on the SH1 Northern Motorway in Auckland, between Greville Rd and Oteha Valley Rd.
"A four-wheel-drive vehicle has left the motorway, crashed down a bank into a couple of lamp posts," a police spokesman said.
"The driver, despite CPR attempts, is deceased at the scene."
Earlier yesterday, two women died in a head-on collision near Thames, on SH25 at the intersection with Pipiroa Rd, between Waitakaruru and Kopu.
A Thames-bound Mazda Demio and a Volkswagen Passat travelling north collided about 7.45am, with the two women in the Mazda dying from their injuries.
It was initially believed both women were Australian, however, police later said it had been established that one of the women was a New Zealand national, and the other was Australian.
Work to formally identify the women and notify their families was underway, police said.
A woman driving the Passat, its sole occupant, was freed by firefighters and airlifted to hospital by helicopter in a serious to critical condition, Sergeant Dave Reid of the Waikato district command centre said.
The busy rural highway, which links Coromandel to Auckland, was closed until mid-afternoon while a lengthy extraction process was completed.
