The Australians were working as military trainers, Cambodian Government spokesman Phay Siphan told the ABC.
Local media outlet Fresh News is reporting that a Cambodian was also killed and two Cambodians were injured in the explosion at a shooting range near a military base on Thursday.
The blast occurred during a joint training exercise at a military base in the province of Kampong Speu, 50 kilometres west of Phnom Penh, said provincial police chief Sam Samoun.
“We don’t know what happened yet, all we know is that there was an explosion,” he said, adding that, aside from the two Australians, Cambodian military made up the rest of the victims.
Cambodian police said a soldier, who was being trained in mine clearance, mishandled a decades-old land mine that had been removed from the ground.
They said a 45-year-old Australian trainer was killed and another 41-year-old Australian was wounded.
The army is also investigating the blast, said defence ministry spokesman Chhum Sucheat.
The Australian embassy in Phnom Penh did not immediately respond to a request for comment.
The injured are being treated at Kampong Speu hospital, east of Phnom Penh.
Some 60,000 Cambodians have been killed or wounded by mines since they were first deployed in large numbers in 1979, when the genocidal Khmer Rouge regime was ousted from power and began 18 years of guerrilla warfare.
Prime Minister Hun Sen said last year that land mines continue to kill or maim nearly 100 people per year, and the country needs more than $400 million in aid to remove all of them by 2025.
Cambodia has cleared about 1,500 square kilometres of mines, but nearly 2,000 square kilometres of land remains littered with the munitions, Hun Sen said.