Four people are dead after car explodes in foggy Waikato smash.

Witnesses to the aftermath of a fiery smash which claimed four lives last night have told of screams for help from two passengers trapped in a wrecked car.

Eight people died on North Island roads in a tragic 24 hours this weekend, four of them in a two-car crash on SH2 near Netherton in the Waikato.

Those on the scene, including a man who rushed to help two trapped in one of the cars, said they encountered a "giant orange fireball" and heard "screams" coming from one of the cars.

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As well as the four people who died, two other people were taken to Waikato Hospital; one in critical condition and another with serious injuries.

Marcia Audain and her partner Roger Farley were heading back to Waiuku when they came across the foggy crash site where a car was on fire.

Audian said a truck yelled to them "I need help, I need help." The next moment the burning car "just exploded."

"It was the worst thing I have experienced," she said.

"The force of the explosion sent the truck driver flying down the bank. That was the moment that happened we realised what we had just come across."

Farley and Audain then realised there was another car down the bank. Farley ran down the bank to the other vehicle and saw there were two people moving.

He forced the doors open and called out for a knife to cut the seatbelts.

"My first thought was get the door open and get them out. I called out for a knife to get the two guys I could see alive out," Farley said.

"They were in a bad way but one said no English and I asked where he was from and he said France."

"I just wanted to keep them talking. I was trying to get the one who was more awake to talk to his mate who wasn't, his eyes were rolling back."

Farley said the time until emergency services arrived was a blur.

"There was four or five of us helping to get them out, some up to their knees in mud. "

Farley said he didn't know first aid so just worked at keeping the men alert.

Aucklander Hamish Anderson, who was travelling to Auckland with his wife, was behind one of the cars involved in the crash.

He said heavy fog meant visibility was reduced to about 20 metres.

"I'm pretty shaken up," he said. "The thought that me and my wife were three or four cars back and another 500m it could have been us. And then seeing people like that ..."

Anderson arrived on the scene seconds after the crash.

"There were screams - not from the car that was in flames - they must have already been dead - but from the car that was in the ditch," he said.

"A few of us tried to get close but the other car was still exploding - literally explosions coming from the vehicle."

Anderson parked his car about 100m back and told his wife to stay in the car and went to try and help.

Fire Service northern shift manager Jaron Phillips said one of the vehicles in the crash was "well-involved" in fire when emergency services arrived.

Two Fire Safety officers were on the scene last night, along with investigators from the police serious crash unit.

Police last night said that fog was heavy in the area and urged motorists to "drive with extreme caution".

Four people died in three other crashes to early yesterday afternoon.

On Friday night a Gisborne woman died after she was struck by a car on Cobden St, Gisborne, just after 11pm.

Two young men - understood to be aged 18 and 20 - died, and another was badly injured after a crash at Dairy Flat, north of Auckland around 1am.

Meanwhile, a woman died in a crash on SH10 at Kaingaoa, north of Kaitaia, about 12.30pm.