Police said emergency services were called to an area of Fitzgerald Avenue near Hereford Street, about 2:15pm on Saturday.
Two people were taken to Christchurch Hospital, while three others in a minor condition were treated at the site.
The young person and an adult were reportedly both in a stable condition on Sunday morning.
A woman who was among the first to the scene said she helped a woman and a child who were seriously injured after a car collided with them at an intersection.
The woman said she helped some men lift the car off a boy trapped underneath
She said her daughter also helped a woman who was the boy's aunty.
Police had earlier asked motorists to avoid the area.
Improvements
A Christchurch city councillor says safety improvements are being looked at for the site.
Central Ward councillor Jake McLellan said the council was looking at roading changes nearby.
"I suspect there there are design improvements that could be made… I imagine that there could be enhancements that we should look at least look at on Hereford Street as well."
Hereford Street intersects with Fitzgerald Avenue.
