Plea to help Sienna Bullock, who survived a crash that killed her mum and twin brother.

More than $21,000 has been raised to support Sienna Bullock, the seven-year-old survivor of Tuesday’s horrific head-on car crash south of Sydney.

More than 300 people have donated to a GoFundMe page established in a plea to help the little girl, who is currently in a stable condition in Westmead Children’s Hospital.

Sienna, her twin brother, Hudson, and 49-year-old mother Julie Bullock were on their way to Wollondilly Anglican School on Tuesday morning when a Holden Astra crossed on to the wrong side of the Hume Highway, striking their vehicle head on.

The Picton mother was pronounced dead at the scene, while Hudson died upon arrival at nearby Liverpool Hospital.

The fundraising page aims to assist Sienna's father Darren Bullock cover the little girl's medical bills, as well as funeral costs for his wife and son.

"To dearest Darren, Sienna and the Bullock Family," wrote one donor, "no amount of money could ever replace Julie and Hudson or ease your pain, but please know you are in our thoughts and we simply wish we could turn back time."

"My heart is broken for you. I knew Julie and she was one of those rare beautiful people who made you feel good just being in her presence. I'm so sorry for your loss," wrote another.

"My deepest sympathy goes out to Darren and Sienna. I lost my father to a road incident 17 years ago and this horrible tragedy has really hit home. I'm so so sorry and feel such sadness for the whole Bullock family," added another.

One bystander claimed to have been holding the primary school student the moment he lost consciousness.

“We were trying as hard as we could to keep the little boy alive and awake but he went unconscious on us. There was nothing we could do,” the witness told The Daily Telegraph.

The 49-year-old driver of the Holden was taken to Liverpool Hospital in a critical condition.

Investigations are ongoing.

To donate to the fundraiser for Sienna and Darren please visit this link.