Senior Constable Cory Ubels of the Serious Crash Unit said a woman was driving towards Hawke's Bay airport approaching a queue of traffic stopped at Pandora Bridge where roadworks were being done.
Temporary traffic signals are operating allowing traffic to cross the bridge on a single lane while work is carried out.
The car crashed into the back of a 300m queue of traffic, hitting the car in front "pretty hard".
Mr Ubels said the car in front of the one hit had heard the approaching car and had the foresight to move out of the queue and on to the side of the road.
This meant it was not also hit from behind.
There is signage alerting drivers to the roadworks but Mr Ubels said the police were investigating if it was appropriate.
A witness to today's crash said they had "seen the crash coming for days" due to the lack of signage in the area.
Travelling that route every day, he had noticed a lot of drivers had to slam on their brakes when they came to the temporary 30km road works zone.
"There is just not enough warning."
The man saw the car slam into the back of the queue.
A hospital spokeswoman said they were treating a five-year-old girl and a woman in her thirties, both with serious injuries.
Another woman in her twenties and her two children were in a stable condition.
The Serious Crash Unit are investigating.
Diversions are still in place between Prebensen Drive and Meeanee Quay for both north and south bound traffic.
Motorists should try to avoid the area.
Roadworks at the site have been suspended temporarily out of respect for the families involved in the crash.
