“It is believed the man had been attacked by the kangaroo earlier in the day,” a state police spokesman said.
An ambulance crew raced to the scene, but the man died there.
“At the property, there was a kangaroo that was preventing the ambulance crew from accessing the injured man,” the police spokesman said.
Police said they had to shoot and kill the animal, as it “was posing an ongoing threat to emergency responders”. The kangaroo was believed to be a wild animal being kept by the man as a pet, police said.
Though the kangaroo species was not identified, the Great Southern region of Western Australia is home to the western grey.
The male western grey can grow up to 2.2 metres (more than seven feet) long and weigh up to 70 kg (154 pounds).
Australian media said the last reported fatal attack by a kangaroo was believed to be in 1936.
In that incident, the Sydney Morning Herald reported that a 38-year-old man, William Cruickshank, had died in hospital in New South Wales, months after being attacked as he tried to rescue two dogs from a large kangaroo.
The attack left him with a broken jaw and extensive head injuries.
(Except for the headline, this story has not been edited and is published from a syndicated feed.)