Niklas Gebhardt has admitted the manslaughter of his 6-year-old son Lachlan in a fiery crash near Rangiora three years ago.

The High Court at Christchurch was told that 32-year-old Gebhardt drove off the road at high speed, with the car becoming airborne and smashing into a tree beside Lehmans Rd.

Lachlan died at the scene. As members of the public dragged Gebhardt out of the burning wreck, he asked to be put back in to change places with his son.

"He's such a nice boy. I want to swap with him," Gebhardt told them. "I want a bullet."

Gebhardt pleaded guilty to the manslaughter charge in the High Court at Christchurch today.

Justice Cameron Mander remanded him in custody for sentencing on August 3. He ordered a pre-sentence report and victim impact statements and gave Gebhardt a first strike warning.

Defence counsel Andrew McCormick said Gebhardt did not seek bail ahead of his sentencing.

Crown prosecutor Mark Zarifeh told the court Gebhardt had shared custody of Lachlan with his ex-partner.

On November 5, 2019, Gebhardt arranged with Lachlan's mother to pick him up later than usual from a swimming pool in Rangiora.

He picked him up from his grandmother at the pool and they left at 4.08pm, with Lachlan sitting in the back seat.

Gebhardt drove at speed along Lehmans Rd, faster than the posted 80km an hour limit. He swerved sharply when he passed a vehicle and had to get back to avoid an oncoming vehicle.

At the end of the straight, there was a right-hand corner with a posted 25km an hour speed advisory.

Police estimated his speed at about 130km an hour when he reached the turn, but he made no effort to slow or swerve.

The car hit the speed sign, went up a bank and became airborne for 24m before it hit a tree 7m higher than its take-off point.

The car caught fire. Members of the public called for emergency services and then pulled open the passenger door and dragged Gebhardt out.

The fire prevented them getting back to the car.

- David Clarkson, Open Justice