An Australian and a Cambodian were killed when a landmine detonated during a military training exercise west of Phnom Penh on Thursday, police say.

Three other men were injured, including a second Australian, though his injuries are said to not be serious. He has been treated at a hospital in Phnom Penh and is able to walk.

The Australians were military trainers, police in Kampong Speu, 50 kilometres from Phnom Penh, said.

Their names have not yet been released.

Police said the mine detonated at a tank base after a Cambodian soldier, who was killed in the incident, picked up the war-era bomb after it had been detected under the ground.

Kampong Speu Provincial Governor Vy Samnang told the Phnom Penh Post it was believed the casualties were shooting at targets around 2pm local time on Thursday when they noticed something on the ground.

"He said they went to pick up the object, a Russian-made bomb, which then exploded," The Post reported.

Cambodia is littered with unexploded ordnance (UXOs), a legacy of three decades of civil war. However, casualty figures have steadily come down over the past two decades thanks to an international effort to rid the country of the blight.

A doctor at the Kampong Speu Hospital, Song Sokvisal, told Fairfax Media an Australian man had arrived at the hospital at about 1.15pm local time with serious injuries to his face, neck and arms.

"His condition was serious. He was in a state of hopeless[ness]. We tried our best, but we could not save him. It happened too fast," the Dr Sokvisal said

It's understood the dead Australian man was 40 and that his body has been viewed by Australian officials in Phnom Penh. The injured Australian man is said to be 39.

Fairfax Media is yet to confirm the identities of either man.

The army was investigating the blast, said defence ministry spokesman Chhum Sucheat.

The Australian embassy in Phnom Penh did not immediately respond to a request for comment.

Comment has been sought from the Department of Foreign Affairs in Canberra.

with Reuters and AAP