Harley said a group of car enthusiasts had met at the site of what's believed to be a new subdivision to check out each other's cars and "network" - sharing tips on paint jobs and vehicle upgrades.
They were on a public road and were not causing any damage to the area, he said.
"All of a sudden this roller starts up."
He thought someone might have been fooling around with the equipment and went to confront them, but soon realised it was nobody from the car scene when the roller began "ploughing through a Mitsubishi Lancer".
The man operating the roller was yelling, Harley said.
People began jumping into their cars and trying to drive away, hitting other people and vehicles in the process.
"It was chaos and this person just kept rolling."
Police said one person sustained leg injuries and another suffered facial injuries after an altercation that ensued.
But Harley said many people were getting hurt.
Harley stayed at the scene to try to make sure everyone got out the way, and saw several near-misses where people jumped out of cars as the roller advanced towards them.
He claimed to see the roller throw a yellow Altezza around "like a rag doll".
Police are now warning against people taking the law into their own hands and seeking retribution.
"We have in place and are working on several more safety measures to ensure the ongoing safety of this subject and his property, due to the threatening sentiments posted on social media," said acting area commander James McKay.
Police said on Facebook they were closely monitoring threatening comments.
"We are also aware of the threats being made to the individuals in relation to this incident," they wrote on the Wellington District Police Facebook page.
"These posts are being monitored closely and will be followed up on.
"We are aware that their information has been plastered all over social media and that is totally unacceptable. The incident has been dealt with ... anyone else that is looking to take matters into their own hands will be looked into."
Harley said the incident was unprecedented.
"In my time I've been out in the car scene, I've never seen anything like that."
He said the group was made up of car enthusiasts and while there was the occasional boy racer that caused trouble, most people were careful to respect property and not do any damage.
Some of their number worked in road works or construction and understood how to protect property from damage, he added.
"Our crew are trying to push more and more young ones to get to the track," said Harley, adding the Wellington car scene had been in place for more than 30 years.
