Two children, aged four and 13, have died after a car crashed into the water at Wyaralong Dam, near Beaudesert in south-east Queensland.
A weeks-old baby, a one-year-old boy and two adults — a man, 23, and a woman, 33 — are in hospital.
The toddler is in a critical condition.
All are from the same family.
Queensland Police Inspector Douglas McDonald said the two adults and four children were in a four-wheel-drive vehicle when it crossed onto the wrong side of the road and broke through a road barrier, rolling down an embankment before ending up on its roof in the dam.
Passers-by, including two off-duty doctors and a critical care paramedic on a day off, helped pull people out of the submerged car.
"It's a tragic and confronting scene," Inspector Douglas said.
"It's absolutely tragic for the family.
"It was fantastic work [by the passers-by] to help rescue the occupants. Their actions were pivotal in making sure this wasn't worse.
"We are still looking into the circumstances which led to the crash.
"At this stage, the vehicle is still submerged, police divers attending.
"It's relatively shallow. The vehicle is on its roof. The wheels can be seen above water.
"We expect to be here until this evening conducting investigations."
Three rescue helicopters and more than a dozen emergency vehicles attended the incident, which happened about 2:00pm near the Overflow Estate Winery at Wyaralong.
One police vehicle flipped as it rushed to the scene, but no officers were hurt.
The two adults in the submerged four-wheel-drive vehicle were transported to the Princess Alexandra Hospital in Brisbane by road. Both suffered leg injuries.
The one-year-old had to be sedated before being flown to hospital, having become agitated after receiving CPR.
Mark Nugent, a senior operations supervisor from the Queensland Ambulance Service, said the actions of the passers-by had helped save lives.
"The six-month-old was removed from the water. There was no CPR [done] on that little one and they were conscious the whole time," he said.
"The emergency response was exceptionally good."
Police said they expected the road to be closed until at least 8:00pm.
