A coroner has found that an Aboriginal man was "cooked to death" after he spent four hours in the back of a security van in searing heat with no air conditioning as it drove across the goldfields of south-west Australia.

The 46-year-old Aboriginal elder suffered third degree burns after collapsing in the heat and falling to the floor of the van while it travelled 250 miles from Laverton to Kalgoorlie in 47C heat.

Ward, whose first name cannot be used because of an Aboriginal cultural prohibition that forbids relatives from naming their dead, had been arrested a day earlier in January 2008 for drink driving.

He was given 600ml (one pint) of water before boarding the van but the coroner found he died before he could finish drinking it.

His body temperature was so high that when he arrived unconscious at Kalgoorlie hospital, medical staff could not cool his body down, despite giving him an ice bath. He also had a cut on his head from falling in the van and a third-degree burn to his stomach from lying on the vehicle's hot metal floor.

The West Australian coroner, Alistair Hope, found that Ward was effectively "cooked" to death and heavily criticised the state prisons department, the private security firm that operated the van and the two guards who escorted Ward.

"It is a disgrace that a prisoner in the 21st century, particularly a prisoner who has not been convicted of any crime, was transported for a long distance in high temperatures," Hope said. The security guards, who did not check to see if he needed a toilet break, food or water, had breached their duty of care.

Even when they heard a thud in the back of the van they failed to check properly and instead threw water on Ward through the chained-up inner door, the Australian Broadcasting Corporation's Four Corners programme reported.

Hope also questioned the reliability of the security guards' evidence and suggested possible collusion, prompting the company that provides the transport service, GSL, to suspend them from duty.

It has been almost 20 years since a royal commission raised the alarm of the widespread lack of care for Australian indigenous prisoners. The 1987 commission noted the disproportionately high number of Aboriginal Australians who were incarcerated and recommended measures to address the problem.

Yet in 2005 a government survey revealed that, while Aborigines comprised 2-3% of the population, they accounted for 20% of prisoners.