A light plane carrying out aerobatic manoeuvres was "upside down" when it hit the ground, killing two men, witnesses said.
Police said two middle-aged men died when the "hobby" plane crashed in farmland east of the Serpentine private airstrip, about 40km south of Perth.
They said they were yet to establish the men's identities and the circumstances of the crash, which happened shortly before 11am (WDT).
Witnesses said the plane appeared to have crashed after diving nose first and "flipping over", ending up on its roof.
"There is debris everywhere, it looks like the plane has disintegrated on impact," one witness said.
Jacqui Lucas, a trainee pilot from Melbourne, told News Limited that she captured the crash on video, which had been seized by police to form part of a report for the West Australian coroner.
She had been filming her boyfriend's plane shortly before the crash happened, she said.
"On the final approach to land, out of the corner of my eye, I saw another aircraft that looked like it was doing aerobatic manoeuvres so I started filming that," she said.
"I just assumed he would pull out of the dive. The aircraft spiralled, went upside down and then I heard a thud.
"At that point because I could not do anything, my attention immediately went to Keiren who was on final approach.
"I was so concerned that he having seen that - I imagined that he had seen all of it - that it would have affected his ability to land safely."
Ms Lucas said the crash was unsurvivable.
"I just thought no-one can walk away from that kind of crash - you can't land upside down and have a good outcome," she said.
A police spokesman would not confirm police were in possession of the footage and suggested it may have been sold to television networks.
Ed Smith, of the Sports Aircraft Builders Club of WA, which operates from the Serpentine airstrip, said the aircraft had crashed in a field near the end of the airfield.
He said the plane did not belong to the club and was not operating from the airfield.
It was believed to be an aircraft known as a Thorpe T-18, which could be kit or factory built, Mr Smith said.
"It arrived over here and was seen to do some minor aerobatic manoeuvres, go out of control and smash into the ground 400 or 500 metres off the end of our airfield," he told Fairfax Radio Network.
"It might have done a loop or suffered a control failure perhaps.
"The plane flipped inverted and came down inverted, petty hard and fast."
He said it was an area where aerobatics were permitted under 3,000 feet altitude and was popular with pilots practising stunts.
Police said an aircraft engineer had been appointed by the coroner's office to assist them with their investigation.