Mr Shortens motorcade had pulled off Cessnock Road seconds before the crash.

He had come to Testers Hollow to make a key election announcement about the flood-prone stretch in a bid to snatch the seat of Paterson on July 2. Instead he suspended his campaign for the day and spent an hour comforting the distraught 33-year-old mother.

Mr Shorten and his team had hurried towards the wreckage and stepped in to support the mother and her shaken toddler.

The politician, who could be Australias next prime minister, cradled the two-year-old boy and put his arm around the mother, leading her to his car where he reassured her. He did not leave her side.

Meanwhile, Labor candidate for Paterson Meryl Swanson comforted the boy in the front seat of Mr Shortens car and read him a small pile of storybooks that ABC journalist Penny Evans had in her vehicle.

Mr Shorten and Ms Swanson later drove the mother and son home and made sure they were comfortable before they left.

The mother sent Ms Swanson a heartfelt text message thanking her and Mr Shorten for their unwavering support.

Mr Shorten took to Twitter on Thursday afternoon to speak out about the ordeal.

Met a brave little boy and his mum today, he wrote.

Days like this give us all perspective. Very glad everyone is OK big thanks to emergency services.

Ms Swanson, who spoke to Fairfax Media late on Thursday, said her instincts took over when she heard the collision on Cessnock Road about 12.45pm.

When I heard the sound my heart went into my mouth, she said.

My first thought was is everyone ok.

Im really happy no one was seriously injured, it could have been so much worse.

Ms Swanson moved towards the mother and her son after she saw emergency services trying to free a 21-year-old woman, who had been driving a blue Nissan Pulsar towards Maitland when the accident happened.

The mother and son had been travelling in a white Mitsubishi Magna behind Mr Shortens motorcade and were heading toward Heddon Greta. They escaped without injury.

The 21-year-old woman was taken to John Hunter Hospital with head injuries and was in a stable condition on Thursday night.

The accident left the road, which is a major thoroughfare to the Hunter Expressway, blocked in both directions.

Ms Swanson told her staff to stop work and reflect on the bigger picture after she left the mother and son at their home.

Images of their distraught faces and the sound of the little boys screams are still firmly on her mind.

I didnt want him to see his mum upset, Ms Swanson said.

[She] was so distraught and the little boy was crying ...She was so worried about her little boy.

Its instinct. You want to put your arm around her and say are you OK, you are OK, youre little boy is OK, no one lost their life, its OK, come and sit down. Anyone would do the same thing, were all human.

Ms Swanson said the little boy soon calmed down and became very inquisitive about Mr Shortens car.

He wanted to sit in the front so I sat with him and read some books to him, she said.

The candidate praised Mr Shorten for his swift response and said the accident had highlighted his true character.

She also said the event had made her even more hungry for the chance to represent the people of Paterson.

Bill was terrific, he said to her to sit in the car, he was fantastic with her, Ms Swanson said.

He stayed calm and reassured her.

Police are investigating the cause of the accident.