At the time, he estimated it to be going at about 20km/h.
Eventually, as the truck neared Rangiriri, a big collision with a barrier slowed it down.
Mr Avatea said he then got out of his car, and went over to truck with another man who had also been following it.
The pair opened the door to the truck's cab, and Mr Avatea jumped inside while the second man applied his hand to the foot brake.
"I jumped into the cab and pulled up the handbrake and the truck stopped,'' he said.
Following this, Mr Avatea - who had done a first-aid course - established that the driver was diabetic and managed to stabilise him.
This afternoon, he was presented with the Governor-General's Anzac of the Year Award by Sir Jerry Mateparae, for his actions in relation to the incident.
The award, which is in association with the Returned and Services Association, recognises New Zealanders who have demonstrated extraordinary comradeship, compassion, courage and commitment, consistent with the spirit of ANZAC.
Mr Avatea and his family flew from Auckland this afternoon for the special ceremony.
"It's been a pretty amazing experience for us.''
They found out he had received the award last week. Mr Avatea said he would take the certificate to a special assembly at his school on Monday.
"I'm not with them [students] at the moment, but they're here with me.''
The family of five, who were also joined by Mr Avatea's mother, Julie, and sister, Melissa Stringer, for today's ceremony, were planning to go out for dinner to celebrate, and visit some of the local tourist attractions in Wellington tomorrow before flying home.
"I don't think in your wildest dreams you'd expect it to be something as big as this,'' he said.
"The whole thing is quite overwhelming because all I did on that day was to help another person who needed to be helped.''
On the morning of the incident, after Mr Avatea had stabilised the man and given his details to police, he and Jordan continued on their way to school.