A male driver of one of the cars was hysterical after the accident. When he learned a child had been badly hurt, he had to be held back by members of the public as he tried to reach an ambulance where the child was being treated.
A woman living opposite the crash site said the noise was "absolutely horrendous".
"I was reading in bed, so I just jumped out because I heard screaming," she said.
"You [could] hear the noise of the tyres, and then the bang it was horrible."
She did not recognise the cars involved, but said some people on her street said one belonged to a neighbour.
Fire Service spokesman Jaron Phillips said the other crash happened at the intersection of Great South Rd and the Southeastern Highway in Penrose just before 6.30pm, and also involved two cars.
Mr Phillips said one person was trapped in a car. The victim was believed to have moderate injuries.
Earlier, commuters faced a slow journey home to West Auckland after a crash on the Northwestern Motorway near Te Atatu. The NZ Transport Agency reported travel times of an hour and a half to get from the CBD to the west.
Meanwhile, a relative of a woman who died in one of the weekend's fatal car crashes said lower speed limits would not prevent such tragedies.
The victim, Judy Redfearn, 68, was fatally hurt near Marton on Saturday afternoon. The Palmerston North grandmother was with her husband, Fred, when their car and a another towing a trailer collided at the intersection of State Highway 3 and Williamson Line about 4pm.
Whanganui residents Zane Town, 14, and his mother Jenny Town, 41, also died.
Police want to talk to the driver of a white van that was at the intersection at the time.
The deaths were among 10 on the roads at the weekend, prompting road policing chief Dave Cliff to suggest some rural roads might need lower speed limits.