A woman died and three other women were injured in a head-on crash in the Coromandel yesterday.

The woman - believed to be 49 and from the Coromandel - died at the scene after her Nissan Terrano crossed the centre line and collided with an oncoming Toyota vehicle on the Kopu-Hikuai Rd, shortly before 3pm.

She was heading east from Thames and the accident happened about five kilometres from the turnoff to Whangamata. She was the only person in the vehicle.

All three women in the Toyota were injured.

A 17-year-old was flown to Auckland Hospital with a serious leg fracture. A second young woman with serious chest, abdomen and pelvic injuries, and an older woman with moderate chest and hip injuries were flown to Middlemore Hospital.

Fairfax journalist Greer McDonald, who was about eight cars behind the Toyota, described the crash scene as "an eerie, silent carnage".

"We knew it was serious because there wasn't much movement. Passers-by were securing the scene, but it was so serious that there wasn't much anyone could do."

An older woman could be seen in the driver's seat of the Toyota.

"She looked alive. She was moving, but she was white as a ghost."

She said both vehicles were severely mangled. "You almost couldn't tell that there were two cars because they were a similar colour and they had just mashed into one.

"It looked like a drink-drive advert. Everything was super slow motion and really quiet. There wasn't a lot of chaos.

"We knew that someone would have died in it."

Fire crews from Tairua and Pauanui were called to cut the deceased and one of the injured from their vehicles.

And two rescue helicopters from Auckland were used to take the injured women to hospital.

Thames senior sergeant Graham Shields said the driving conditions on the Kopu-Hikuai Rd were "horrendous" following persistent rain overnight on Saturday and yesterday morning.

"Very wet, a lot of rain, a lot of surface flooding and greasy roads," he said.

Mr Shields said Hamilton Serious Crash Unit was investigating the accident last night, but police were confident that the woman who died had crossed the road, into the path of the Toyota.

"It's a high-speed, head-on crash," he said.

"It's pretty serious. People have just got to drive to the conditions. When it's wet, slow down."

State Highway 25a was closed for over four hours last night and traffic was diverted through Whangamata, creating a lengthy detour for northbound traffic.

Ms McDonald was the first on the scene at another crash about 20 minutes earlier after a station wagon hit a bank and flipped on to its roof just south of Whitianga.

The driver, a young man, had to crawl out of the passenger window, but seemed unhurt.

- © Fairfax NZ News