On Monday, the male driver was in a critical condition at Westmead Hospital in Sydney, having been airlifted from Batemans Bay Hospital on Saturday afternoon after suffering head injuries in the crash.

Police said the family had been driving east along the Kings Highway near Braidwood when the car started to come off the road. The driver had tried to correct the van before it came up a wire safety barrier, hitting a guardrail and flipping, landing on the driver's side before resting on its roof.

Passers-by stopped and righted the vehicle – flipping it back onto its wheels – before they pulled the mother and three children from it. They were unable to pull out the 42-year-old father, who remained trapped in the car until NSW Fire & Rescue freed him.

Emergency services said they responded to a single-vehicle crash about 12.40pm on Saturday after bystanders had already freed four of the car's five occupants.

The 39-year-old woman, now discharged from hospital, was treated at the scene before being flown to Sydney to be with the family.

The three children were taken to Westmead Children's Hospital; two of them were airlifted with serious injuries but as of Monday they were both in stable conditions. It is unclear of their ages or genders.

The third child, a 7-year-old boy, was taken to Bateman's Bay Hospital before being airlifted to Westmead with his mother. He was in a satisfactory condition on Monday.

NSW Police Inspector Angela Burnell urged motorists to take care when travelling on the Kings Highway.

"It can be a challenging drive, if you're unfamiliar with the conditions allow yourself extra time," she said.

with Finbar O'Mallon