Tanja Petrovic was driving on the Hume Highway on her way back to Canberra on Monday afternoon when she saw a plume of smoke ahead and the crumpled four-wheel-drive and caravan.

The vehicle had rolled after hitting a guard rail on the Hume Highway bypass, trapping a Queanbeyan woman inside and injuring her husband and children.

Ms Petrovic said when she pulled over, the father handed her the six-month-old baby as he ran to the wreckage to free his second child.

She wrapped her cardigan around the baby and held him close.

"The baby was very, very distraught, crying. He had no bleeding anywhere but he had a broken arm. You could tell the arm was broken by the way it was wobbling and he couldn't lift it up," Ms Petrovic said.

The father, who had escaped with minor head injuries, managed to retrieve his three-year-old son and handed him to a second bystander as he went back into the car for his wife.

Ms Petrovic described the little boy as "very brave".

"He kept saying he was better, he wasn't hurt and he kept talking about Spiderman to me. He just wanted to have a conversation about Spiderman," she said.

Emergency services frantically worked to free the woman, who was seriously injured in the crash.

Ms Petrovic said the father refused to leave his wife's side.

"He was very distraught, very worried but he kept calm. He was coming in and out of shock," she said.

Cars kept rushing by as Ms Petrovic consoled the tiny baby.

When the father came and took the baby to the ambulance, Ms Petrovic scooped his older son up and took him to see his mother before she was airlifted to Canberra Hospital.

"She was put on the stretcher and I told her he was doing fine, he was with us," she said.

A mother herself, Ms Petrovic said she wasn't panicked by the scene.

"I just wanted to see if the kids were okay first and if they needed anything, clothes or water or anything like that," she said.

But she condemned the callousness of people who drove on without stopping to see if the family needed help.

"There were onlookers that didn't stop, they just kept driving. After five minutes they got in their cars and left, we were the only ones who stayed."

A statement released shortly afterwards by the family via social media said they were thankful to everyone who helped to keep their children safe.

A police investigation into the crash is underway.

South-bound lanes of the highway were closed with long weekend traffic queued up to 3.5km for much of the afternoon.