Two children have died, trapped inside their family's semi-submerged car after it crashed through a barrier and flipped upside down into southeast Queensland's Wyaralong Dam.
Four other family members escaped the vehicle, which rolled down an embankment before coming to rest on its roof.
The girl, 4, and boy, 13, were travelling in the four-wheel-drive near Beaudesert about 2pm on Sunday when it overturned at high speed.
A man, 26, and woman, 33, were able to free themselves along with a five-week-old girl and boy aged one from the Landrover Discovery.
But the couple struggled to get the doors open to reach the older children, Inspector Doug McDonald told reporters on Monday.
"Unfortunately, those two children were trapped in the vehicle longer than the other occupants," he said.
"There was some difficulty getting some of the doors open ... to extract the kids.
"Once they were taken from the vehicle (there were) attempts for quite some time to revive those children."
Insp McDonald said the car veered into a barrier on the left side of the road at high speed then "skewed across to the opposite side".
"Consequently the angle it's gone into that other barrier has resulted in it flipping and rolling down that embankment into the water," he said.
Two children were pronounced dead at the scene.
The surviving children remain in hospital. The boy is in a stable condition and the girl suffered no serious injuries.
The man and woman suffered non-life-threatening injuries and will undergo surgery on Monday.
Insp McDonald said the scene was "really traumatic" for first responders and passers-by who stopped to help.
The family were travelling from Pimpama, on the Gold Coast, to their home in Pratten, near Warwick in the Southern Downs Region, when the accident happened.
Alcohol doesn't appear to have been a factor and the car was travelling on a straight section of road with a 100 km/h limit.
A police officer suffered minor injuries when the police car he was traveling to the dam in on Sunday rolled onto its roof on an unsealed road.
Police appealled for anyone with dashcam footage from the area on Sunday to contact Crime Stoppers.