The man said he was the first on the scene, near a roundabout at the intersection of Ruapehu and Tamamutu streets.
When he got there the car was still in drive, with the driver's foot on the brake.
He told the Daily Mail that he put it in neutral and pushed it away from the roundabout.
"It was outside a school and it's definitely a wake-up call.
"It could've gone totally wrong," he told the Daily Mail.
The video, which has been shared numerous times on Facebook, was reportedly filmed on December 6 just after 3pm.
The video is the latest to emerge showing the aftermath of people using the powerful synthetics - labelled "zombie drugs".
In September New Zealand Police said about 20 people were believed to have died as a result of synthetic drugs.
Detective Inspector Scott Beard said at the time the substances were very dangerous, given that they could come from anywhere.
"It is a dangerous drug that we know is a synthetic compound usually manufactured overseas.
"Those taking it, are taking a huge risk because you do not know where it has come from, or the level of dosage," Beard said in September.