Leon Kang was one of the motorists who stopped when he saw the black cloud stretching out in the sky.
Kang said traffic slowed to make way for emergency services as firefighters rushed towards the flames.
He stood back about 30 metres and could feel the heat from the flames as the truck burnt to the ground, Kang said.
The tyres exploded under it, he said.
The Caltex service station manager said the truck had caught fire behind the station at the truck stop.
A former firefighter had seen the smoke and stopped to help, he said.
The fire was nearly 500 metres away from the petrol station forecourt, he said.
However, the store had closed for an hour as a safety precaution, he said.
"Everything is controlled right now," he said.
There were plenty of firefighters, traffic controllers and police at the scene, as well as Auckland Council representatives, he said.
A Fire and Emergency New Zealand spokesman said they were called at 10.23am to reports a truck and trailer containing cardboard was on fire.
It appeared it had gone up pretty quickly, he said.
They received multiple calls because the service station is close to the Thermal Explorer Highway, he said.
Five fire engines attended the initial callout and were joined by two water tankers.
The two water tankers were still on the scene at 12.30pm.