The Beech Bonanza crashed south of Brisbane about 10 minutes after taking off from Archerfield Airport about 11am on Monday, police said.
"As to what happened whilst that aircraft was in the air, we have no information at this time," Chief Superintendent Mel Adams told reporters on Monday.
"It was a very confronting scene, as you can imagine."
Crews called to the grass fire found plane debris scattered on the Stockleigh property and flames racing across an area about 300m by 400m, authorities said.
Multiple crews worked to quickly contain the blaze, which burned close to a farmhouse before being brought under control.
No homes were damaged.
"At this really early stage in the investigation, I can't comment on the persons within the plane," Chief Superintendent Adams said.
"Our notification of next of kin is still under way.
"As you can understand, this is a really sensitive matter, and we won't comment further on those persons that lost their lives today."
The Australian Transport Safety Bureau confirmed it had begun an investigation with a three-strong team specialising in aircraft operations, maintenance and engineering sent to the site.
They will gather evidence and examine the site and wreckage, recovering any aircraft components of interest for further examination at the bureau's technical facilities in Canberra, it said.
"Investigators will also interview any witnesses and involved parties, and collect relevant recorded information including flight tracking data, as well as pilot and aircraft maintenance records, and weather information," it said in a statement.
A preliminary report on the crash was set to be published in about eight weeks.
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