It is understood her mother had just given her 20c and the child had slipped away to visit a nearby shop.
Police said the girl was walking on a pedestrian crossing controlled by traffic lights at the time. She had pressed the button and waited until the "green man" signal showed, meaning she could cross.
The girl's aunt, who the Herald has agreed not to name, drove past her just seconds before the youngster was hit.
As she looked in her rear-view mirror to see if the girl had made it across the road safely, she saw the impact and her niece flying through the air.
The aunt was too shaken to speak about what she saw, but her husband told the Herald: "My wife was just about to wind down her window and say 'make sure you cross the road properly', but she had to turn the corner.
"My wife had me on speaker phone because I was giving her directions to a street. All of a sudden she screamed 'Oh my God' and hung up.
"My wife saw her getting hit by the car in her rear-vision mirror. Her niece went up in the air about 2.5m off the ground and then came down and landed."
The aunt pulled over and ran to the girl, who was unconscious. A couple who saw the accident also stopped to help.
They were in their truck waiting on the flush median to turn right on to Croydon Rd.
"The girl was basically running across the road, she had the green pedestrian sign. The lady in the other car didn't see her," the man said.
"When she struck her, the girl flew, she travelled about 30m in the air from where she was hit ... it wasn't nice."
The man jumped out of his truck and ran to the injured girl. "She was out of it, she wasn't responding or anything for about five minutes, then she came around slowly and started to groan.
"She definitely had a head injury and her left leg was broken, it was like rubber."
The woman whose vehicle had struck the girl also got out of her car.
The man was "really affected" by the incident and said the intersection was dangerous. "It's terrible. They need to do something about it.
"There's so many different roads coming off it and Titirangi Rd is chocka with traffic.
"I really hope that little girl pulls through."
Her parents were with her at the Starship hospital.
The impact was so hard that the girl's shoes came off. Her uncle said she suffered multiple injuries.
She had stayed home from school yesterday, and was with her mother just before the incident.
"She was playing and she asked her mother for some money. Her mother gave her 20c and then she went to play outside. Then, all of a sudden she was missing.
"She must have been on her way to the shops."
School: 'Hoping for the best'
The school the girl goes to has posted a message on its Facebook page, saying it was "hoping for the best" for its pupil.
Fruitvale School said "one of our wee Fruitvalers was knocked over" yesterday morning. She was not on her way to school at the time, it said.
"Latest update is that she has a broken leg but appears not to have any internal injuries or head injuries," the Facebook message said.
"She is still classed as serious but we are all hoping for the best. Our love goes to her family."
The message was posted yesterday, and attracted a number of well-wishers, sending the girls get well soon messages and "love, hugs and prayers".
On woman, writing on behalf of her son who also attends the school, said: "All the best for a speedy recovery."
Another said: "Wishing her all the best. Such a sad thing to happen. Sending all our love, thoughts and prayers."
The Waitemata serious crash unit and Henderson police were investigating the incident. The female motorist had been interviewed but it would be several weeks before police made a decision on whether she would be charged.
