Inspector Douglas McDonald said the car had been travelling along a straight stretch of Beaudesert Boonah Road about 2pm when it veered to the left and crashed into a guard rail.
Inspector McDonald said the vehicle then skidded onto the other side of the road and crashed into a second guard rail before rolling down a stone embankment.
"It came to rest on its roof in a semi-submerged position," he said on Monday.
Inspector McDonald said the two adults extracted themselves and some of the children from the car.
"Unfortunately, those two children (who died) were trapped in the vehicle longer than the other occupants," he said.
"There was some difficulty getting some of the doors open to the vehicle."
Inspector McDonald said passers-by performed CPR on the children once they had been removed, but they died at the scene.
"It's really traumatic for our first responders and also for the members of the public who stopped," he said.
Inspector McDonald said investigators still didn't know what caused the crash, but alcohol was not believed to be involved.
Child seats were also found in the vehicle, and all occupants were understood to be wearing seatbelts.
Inspector McDonald said the family, from Pratten near Warwick, were travelling home from Pimpama on the Gold Coast.
A five-week old baby girl, one-year-old boy, 33-year-old woman and the 23-year-old male driver are all in a stable condition in hospital. Photo / Channel 9
"Fatigue is a really big killer on our roads, and that's certainly part of our investigation for this matter," he said.
Police have spoken briefly to the male driver and female passenger, but formal conversations are yet to take place.
The pair is expected to undergo surgery on Monday for non-life-threatening injuries.
Millions of people in Queensland and New South Wales have been warned to brace for days without power after Tropical Cyclone Alfred makes landfall later this week.
