A man who arrived at the scene just after the girl was hit said passerbys put the girl in the recovery position.
A teacher from her school and a nurse were amongst those to arrive first.
The driver of the car - a woman believed to be in her mid to late 20s - was inconsolable, said one man who wished not to be named.
"She was hysterical, saying if the girl is dead she couldn't live with herself."
Balmoral Police Sergeant Jason Loye said early indications were the driver was distracted.
It was not clear how she was distracted.
"She's distraught and is giving a statement at the moment," he said.
It was a harsh reminder for drivers to pay complete attention to driving when they are behind the wheel.
The section of road remained closed as police crash investigators studied the scene.
The car was stopped less than a car length past the median island where the girl was believed to be crossing from.
The median island has a division for pedestrians to cross from.
A spokesman for the hospital said the girl was in the emergency department after she was rushed to Starship by St John Ambulance service in a critical condition.
A person at the scene said the girl was crossing at a traffic island, rather than at the lights, when she was struck by a dark coloured Pugeot hatchback.
He said the car was still at the scene and he believed the girl had been thrown "a distance" upon impact with the car.
"Police are talking to students who look quite visibly upset," he said.
Police said St Lukes Rd was closed to traffic heading east, which was being diverted along Taylors Rd.
"Only one lane is open in the westbound direction and drivers are advised to avoid the area as there are delays."
It was not known when the road would be cleared, police said.
