Emergency services were called to the fire at about 3pm.
Two fire trucks from Alexandra attended, which drew onlookers from the council buildings, the adjacent police and court buildings, as well as the nearby primary school.
Chief Fire Officer Russell Anderson, of Alexandra, said the vehicle belonged to a member of the public.
Two trucks had been sent as a precaution.
CFO Anderson said the fire was likely due to a fault in the engine.
"You've obviously got a spark of some sort that's started it and you've got a source of fuel to get it going the way it has," he said.
A fire investigator had been notified, he said.