Serious Crash Unit investigations indicated the boy was in the driver's seat of an older-model sedan, when he released the handbrake, causing the car to roll backwards down a steep driveway, Mr Garnett said.
"It appears the boy either fell out of the car or tried to get out and then was hit.
"The boy's mother was at the house at the time, while his father was at a neighbouring property. The family was "understandably distraught", Mr Garnett said.
"They're going through a horrible time in their lives and we just offer what support we can and just be there for them," he said.
A large group of family and friends was arriving to surround them with love and support, Mr Garnett said.
"They've come from all over the place, and they've both got a strong group of close friends in town which are starting to congregate as well, which is pretty cool."
He added: "You're not supposed to bury your kids. [It's] very tragic, very sad."
No charges will be laid in relation to the death. Police will prepare a file to be given to the coroner.
Safekids Aotearoa director Ann Weaver said yesterday's fatality was another sad reminder to be vigilant with children around driveways.
"It is very sad to hear of this, it is so sad and our thoughts are with the family who have lost their child," she said. "It is that time to remind parents to be extra vigilant with children around vehicles."
In 2005, siblings Hayley Jayne Keenan, 20-months, and Cory Andrew Keenan, 4, died after the double-cab utility they were in alone plunged down a steep hill on Mt Hyde Station, near Hindon, 40km northwest of Dunedin.
Their father, Andrew Keenan, told police the children had been playing in a paddock while he was digging out thistles on the property. The ute was parked, with the gear-stick in reverse and the handbrake on, but he turned around to see it moving. He chased the vehicle but was unable to stop it or resuscitate the children.
- Additional reporting: Morgan Tait