A young boy who was killed when his family van rolled and flipped on the Southern Motorway yesterday may not have been buckled in.

The 6-year-old was trapped underneath with another relative, and died at the scene. Six others were in the vehicle, some of whom were thrown clear of the wreckage.

Police said it appeared the boy was not restrained, but a full investigation would be carried out to confirm that. If so, charges would be laid.

His death was one of three on the roads yesterday.

The child was travelling with extended family in the southbound 1994 Toyota Emina Estima when it spun 180 degrees and flipped, landing on its side just past the Orams Rd overbridge at Manurewa about 1pm. No other vehicles were involved.

Constable Paul Hayward, of the Waitemata serious crash unit, said the cause of the crash was not yet clear, but it could have been a result of speed or fatigue. Tests to determine whether alcohol was involved were not yet back.

Mr Hayward said there were two curved friction marks, or "yaws", on the road where the car had spun out before hitting the barrier and coming to a rest on its side.

It is understood the people-mover van bore members of a Tauranga family who were heading home.

A resident on the family's street was devastated to hear about the crash, but said she often saw the children in the van unrestrained.

"Preschool kids in the front not even in car seats," she told the Herald.

Middlemore Hospital spokeswoman Deborah Paget said one female passenger was to undergo surgery on her arm at Middlemore Hospital last night and three other women were in the ward or under observation. She said a 2-year-old boy, another woman and a 20-year-old man had been discharged.

The accident brought traffic to a standstill for hours on the Southern Motorway.

Vehicles were backed up from Orams Rd to Highbrook Drive at peak - with police diverting motorists from Te Irirangi Drive.

Northern Communications Inspector Cornelius Kluessien said some drivers verbally abused road staff putting up barriers to protect the crash scene. "You've got to wonder about that attitude of some people," he said.

Officers from the Manukau and Waitemata serious crash units were at the crash site until the motorway was re-opened about 4pm, examining markings, making calculations and taking photographs.

Road workers put up barriers on the sides of Orams Rd overbridge to shield the view of the scene from pedestrians and motorists.

- additional reporting NZPA