The death is the first in the festival's 29-year history.

The organisers were unaware of the seriousness of the incident until Friday afternoon, when the death was reported.

It is understood the man fell off the back of a flat-tray ute about 4pm on Thursday. He died later in the Canberra Hospital as a result of his injuries.

ACT Policing would not release further details about the incident on Friday other than to say it involved a single vehicle.

"This is the first person to have died on ACT roads this year," police said, adding that the collision investigation and reconstruction team would examine what happened.

In a statement, the festival's organisers offered their deepest condolences to family and friends of those involved.

The event's founder and former promoter, Chic Henry, said on Friday evening he was devastated.

"I'm extremely sad for the guy, and for everyone," he told Fairfax Media.

"It was always in the back of my mind that, one day, this would happen. And here it is. It's very sad."

Mr Henry hoped the tragedy would not stop everyone from "having a good time" at the festival.

"This should be an enjoyable time. It's just devastating."

ACT Policing said it would prepare a report for the coroner.

A Summernats spokesman said he did not know where the man was from.

"We really don't know much at this stage, which is why we are looking for witnesses. We want anyone who saw anything to please come forward and contact us or police."

An ACT Policing spokeswoman said case officers had been in contact with the family.

The Summernats festival is Australia's best known car festival, drawing crowds of more than 100,000, and running annually in Canberra since 1987.

In 2006 four people were injured when when a ute performing in a stunt show crashed through the perimeter fence.

And in 2000, a security guard was hit on the left leg by a car while working at Summernats.

The festival has also been marred with controversy over the years in relation to antisocial behaviour and sexual harassment.

In 2008, The Canberra Times reported hundreds of men marched around the festival's main cruising route harassing women with demands to get their breasts out.

Uniformed police began patrolling inside Exhibition Park during Summernats in 2012 for the first time in nine years, targeting hoons, violence and anti-social behaviour.

The onus for maintaining security previously rested primarily with organisers and their contracted security guards.

Also in 2012, Summernats organisers pledged to ban for life a group of men involved in an ugly fracas with security staff after an illegal burnout.

However, in recent years police had made minimal arrests and praised the behaviour of the majority of the festival's crowds.

The event will run as scheduled on Saturday, and its organisers will hold a press conference in the morning to discuss Friday's death.