Police said it appeared the boy had run out onto the road in front of the car.
It is understood the 7-year-old's family had been informed but his name had not yet been released.
After the boy was hit teachers from nearby Cannons Creek School, which had just released its pupils for the weekend, ran out on to the road to help him.
The boy was a pupil at the school and staff there refused to comment as it was "too soon".
Elizabeth Tuiala, who across the road from the fatal scene, had just walked outside when the boy was hit. She said there were a lot of children in the area at the time.
She said the car appeared to be going "fast".
Senior Sergeant Derek Chapman of Porirua police said the incident was a "tragedy" and a reminder for all motorists about children not being able to judge speeds and distances of cars the way adults did.
"It's just another reminder to drivers to look out for kids around schools and slow down," he said.
Police would visit the school on Monday to speak to staff and pupils.