A Northland man is believed to have been drink-driving and travelling at high speed moments before a fiery crash which left a woman and one of her three children in the other vehicle critically injured.

The crash occurred on State Highway 1 near the bottom of Turntable Hill, Moerewa, at 7.35pm on Sunday.

Witnesses sat a BMW involved in the crash passed through Moerewa township at high speed.

Early yesterday the 33-year-old woman, who lives in Auckland but has close family ties to Northland, and her 4-year-old son were both still in a serious condition, the mother in Auckland City Hospital and the boy at Starship.

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Two other children, aged 7 and 9, were also taken to hospital, along with two passengers in the offender's vehicle. The driver, a 31-year-old man from Moerewa, initially fled on foot but handed himself in at Kawakawa police station on Monday morning.

Police Inspector Chris McLellan said the Serious Crash Unit was still investigating but it was believed a northbound vehicle had travelled through Moerewa at excessive speed before losing control at the last corner as it exited the town, skidding sideways into the path of a southbound car. The impact was severe, he said.

''This was a lovely family returning from a family event in the Far North ... they were hit by an out-of-control vehicle travelling at excessive speed.''

The driver of the north-bound vehicle was also believed to have been under the influence of alcohol.

McLellan said he couldn't praise the Moerewa community enough for their response after the crash.

''People came out of their houses to help us on the cordons, they supported the injured people and allowed their houses to be used by emergency services ... Another indication of the strength of the Moerewa community was how quickly police were given the information they needed to identify the driver,'' he said.

The crash had been tough on emergency services because three of the injured were young children. No charges had been laid as yet.

Police had been out in force during Labour Weekend trying to persuade drivers to be conscious of speed and avoid drink-driving.

''This crash was a consequence of both those things and some really poor decision-making,'' he said.

Police, St John Ambulance, volunteer fire brigades from Kawakawa and Kerikeri, and rescue helicopters from Auckland and Whangārei responded.

Kawakawa deputy fire chief Alistair Leitch said the initial 111 call was for a car versus a pole. While the first crew was on its way to Moerewa they were told it was a two-car crash, and when they arrived they found one car was on fire.

Once they were sure no one was in the burning BMW they concentrated on freeing the woman trapped in the other vehicle. When emergency services arrived the children were being looked after at the nearest house.

''There was a lot of support for them coming from the crowd and nearby houses. Bystanders did a really good job,'' Leitch said.

When Kerikeri firefighters arrived they doused the burning car and set up a landing area for the Westpac Rescue Helicopter at the junction of SH1 and Snowden Ave. The Northland Rescue Helicopter collected the injured boy from Bay of Islands Hospital.

Part of the investigation will involve calculating the speed of the BMW before the impact.