"I couldn't say how steep [the driveway] is but it's a reasonable incline," he said.
"It appears the boy either fell out of the car or tried to get out and then was hit."
Police were yet to confirm whether the boy's brother was in the car, Mr Garnett said.
"It seems that he wasn't but we're not 100 per cent sure at this stage."
The mother was at the house at the time of the accident, while the father was at a neighbouring property.
The family were "understandably distraught", Mr Garnett said. No charges would be laid in relation to the accident.
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.
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."
Driveway Deaths in NZ
• 5 children are killed on average each year in New Zealand driveways
• About 1 child a fortnight is admitted to Starship hospital with injuries from a driveway accident
• Half of drivers are the victim's parent
• Most children injured are aged 2
• Accidents frequently happen in the summer months
How to stay safe around driveways
• Identify risks eg. long driveways, one that provides pedestrian access to a house, near lots of parking
• Check for children before driving off
• Supervise children around vehicles
• Separate play areas from driveways
- additional reporting Morgan Tait