Evidence from the scene of the Picton crash that claimed seven lives indicates that driver fatigue or distraction are likely suspected causes, an expert investigator says.

A family was returning home to the North Island after attending a loved one's funeral in Dunedin when the Toyota Hiace van they were travelling in collided head-on with a refrigerated truck on Sunday.

Experienced vehicle crash investigator Hamish Piercy told Newstalk ZB's Tim Dower that police said the van was on the wrong side of the road at the time of the crash.

"There's a straight leading into that bend and it looks like the van has simply gone straight ahead at the end of that curve which suggests to me potentially either a fatigue or distraction-type crash," he said.

There were nine people in the van and tragically seven of them, including an infant, were killed "in the blink of an eye" after their van appeared to cross the centreline on State Highway 1 south of Picton about 7.30am. Two survivors are in Wellington Hospital and the truck driver was released from hospital last night.

The huge collision tore the van open and left the mangled vehicle completely destroyed. Photo / George Heard

Piercy spent 16 years with NZ police and is an expert in crash investigation and analysis.

He said fatigue was more than just about going to sleep behind the wheel.

"It's about a reduction in their ability to drive safely; they start cornering late, they brake late or their perception about how they are driving is not necessarily great, their vision and focus becomes tunnelled focus as they start to become more and more tired and the driving becomes less smooth," he said.

It was also about the circumstances leading up to the crash.

"You have to look at when the van started the journey, what the circumstances were, how long had they been driving."

Two people from the van were flown to Wellington Hospital with serious injuries, while the truck driver had minor injuries. Photo / George Heard

Marlborough District mayor John Leggett told RNZ the accident was an "absolute tragedy".

He had heard the sirens responding to the crash.

Leggett said that section of the state highway was one he knew very well.

"On the face of it there doesn't seem to be any obvious roading issues, or traffic issues."

If safety improvements were required, the council had been assured they would be implemented without delay, he said.

"There will be a full report done and we will expect appropriate action to happen."

Meanwhile motorsport legend and road safety campaigner Greg Murphy said the country was spending millions of dollars of taxpayer funds in "horrible advertising campaigns" around driving rather than better training for those behind the wheel.

"[They] don't make an iota of difference," he said of the advertising campaigns.

Murphy told Newstalk ZB's Mike Hosking that the country was "stuck in the same patterns" it had been in for decades around driver training.

He believed officials were "scared to give people a better base level" of understanding of what's needed when driving.