Off-duty Matt Patterson wearing a red T-shirt is seen rushing to the aid of the little girl, who lost a leg in the attack and remains hospitalised in a critical condition.
Jane is seen sitting up and then lying in the middle of the road, moments after standing on the sidelines where she was photographed alongside her brother Martin.
Following the bombing, others are filmed standing around stunned but Matt, who had been enjoying a meal in a restaurant with his girlfriend, leaps into action.
Just before the explosion Matt had been enjoying a meal with his girlfriend in a nearby restaurant.
"This child for some reason just jumped at me - it was tunnel vision," he said.
"I could see the damage. I could see that she needed a tourniquet. She needed surgery, it was obvious from what I saw.
"I go up to a business guy and ask for his belt, he hands it over without hesitation. I have to stop the bleeding. I am making the tourniquet and another man comes and I ask him to hold it for pressure.
"I'm looking up and down the street - looking for where I can bring the child to an ambulance or more care."
Matt and the other man ran with Jane to a waiting ambulance. She has lost a leg and remains in a critical condition in hospital but is expected to survive her injuries.
It has now emerged she was an enthusiastic Irish dancer having attended classes since the age of three. Several Irish dance groups are now raising money to support her.
Her eldest brother Henry, 12, and father Bill managed to escape from the bomb attack uninjured.
Images from the scene appear to show Bill, 42, a community organiser, holding his youngest children up on the railings.
Their mother Denise, 43, sustained head injuries and had to undergo brain surgery following the attack.
On Tuesday the family held eight-year-old Martin's funeral.
In a statement they said: "The outpouring of love and support over the last week has been tremendous. This has been the most difficult week of our lives and we appreciate that our friends and family have given us space to grieve and heal.
"A private Funeral Mass was celebrated this morning with immediate family. We laid our son Martin to rest, and he is now at peace. We plan to have a public memorial service in the coming weeks to allow friends and loved ones from our community to join us for a celebration of Martin's life."
The bombing killed three and injured more than 200, 48 of whom were still in hospital yesterday, three in a critical condition.