The agency which was helping the intellectually disabled man who was fatally injured when he fell from a moving van says it will now review its services.

The middle-aged man died at the scene after he fell from the van on Auckland's Southern Motorway just after 11am on Thursday. The van had been travelling at 100km/h.

He was a client of Framework Services Ltd - a subsidiary of the Framework Trust - which provides community based services to support people who are mentally ill or have intellectual disabilities.

Its chairman, Fuimaono Tuiasau, said today: "Framework Services are cooperating with authorities' inquiries, and we will be conducting an internal review of the incident and our procedures to improve if necessary our practices and processes."

The scene of fatal incident on Auckland's Southern Motorway at Ellerslie in which a man died after falling from a moving van. Photo / Doug Sherring

The police are investigating the incident on behalf of a coroner.

A woman who saw the fatal event unfold from two cars back told the Herald: "It was unbelievable how the man didn't get run over by the truck that was on the left lane as it happened so suddenly."

All citybound lanes on the motorway were closed after the incident, which happened near the Ellerslie-Panmure interchange, from 11.15am to 1.30pm. Traffic was backed up as far as Sylvia Park.

Some drivers reversed back along the motorway to try to avoid the crash scene.

A police helicopter landed in the middle of the motorway to provide immediate first aid.