A motorist who stopped at a crash in West Auckland that killed a pedestrian has hit out at "bystanders" for not stopping to help.

The driver, who pulled over to help a critically-injured pedestrian who had been hit by a car on Te Atatu Rd on Tuesday night, was shocked several others drove past the fatally injured woman without stopping.

The man, who did not want to be named , was driving past the incident about 6.30pm when he spotted the woman lying on the road.

"From a distance I thought it was a rubbish bag. She was wearing all black, but when I got closer, it was a person," he said.

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"I pulled over to see what is going on and it looked like she had been hit by a car. I moved her on her back and she was unconscious."

The man said about seven or eight cars also drove past the woman but no one stopped to help.

"They all slowed down and saw her on the floor but no one could bother to stop. I'm not sure how long she had been there for, but I'm sure somebody could do something or at least call the ambulance," he said.

"That lady is somebody's family member and it could be one of ours. Life is too short and this could happen to anyone. What if she was your mother or your sister? Would you want people to treat her like that? Please look after each other. Karma is just a step away."

The man was about to call an ambulance but there happened to be one driving past so he waved it down to assist. They didn't know anything about the crash.

He said the paramedics tried for about 40 minutes to revive the woman.

St John said they did receive a call about the crash shortly before the first ambulance arrived.

Police announced later that night that the pedestrian had died in hospital.

A police spokesperson said the pedestrian suffered critical injuries in the collision, which occurred on Te Atatu Rd at around 6.40pm.

Police said the driver also stopped at the scene and was working with police.

"It is hard to see something like that happen to anyone out there, but what's harder is people have gone sour these days," the witness said.

Danny Osborne, lecturer in psychology at the University of Auckland, said the incident showed a pretty clear example of the bystander effect.

"We like to believe there is safety in numbers, but we are actually probably less likely to receive assistance when we are in a large group rather than if there is only one or two people around," he said.

"It is almost like there is a diffusion of responsibility when you are in a large group, so each individual feels less responsibility and we assume someone else is going to act rather than ourselves."

Osborne said a similar phenomenon would have affected the people who failed to stop at the crash scene.

"When you are in your car and driving by, you assume someone else is taking responsibility.

"Te Atatu Rd is a pretty busy street, so just knowing that so many cars are passing by could be enough to make people feel less responsible for stopping," he said.

"It very well could be that they saw this good Samaritan helping the pedestrian who had been hit, and assumed someone else was already taking care of it.

"It is not that the individuals that didn't stop are bad people, it is just the power of the situation is really driving behaviour here."

Osborne said there were multiple stages people had to go through when deciding to help someone, and if you understood the stages better you were more likely to help.

"The first stage is noticing that there is a problem unfolding in front of you - it sounds like in this instance most of the passers-by did notice.

"The next stage is interpreting that it is an emergency situation – again the passers-by presumably saw the person on the side of the road and interpreted it as an emergency.

"The next stage is responsibility and that is where I think things went wrong here and people didn't take responsibility for helping," he said.

"After the responsibility phase then there is the decision stage, so people weigh up the pros and the cons, is it worth it for them to stop, and whether they have the skills to help. The final stage is stopping and helping."

St John Assistant Director of Operations Tony Devanney said generally the New Zealand public were very willing to help and act as first responders.

"As an emergency service we owe them a great deal of gratitude," he said.

"Where this doesn't happen can often be attributed to a fear or lack of confidence about not having the right skills or first aid to help. We recommend all New Zealanders take a St John First Aid training course to acquire the skills, and more importantly, the confidence to stop and help.

"We thank the motorist who did stop and remained with the patient, keeping them safe until our ambulances arrived."

Police declined to comment further on the incident while the Serious Crash Unit was investigating.