"There was a crowd gathering and we were concerned for the safety of some of our equipment."
Police were called to guard the trucks shortly after midnight and Bell said he did not believe anything had been stolen.
"It was more of a preventative measure," he said.
"Based on some previous trouble around the rest of the country, particularly in student areas, it is a concern we had."
Bell was unable to confirm reports items, including road cones, had been stolen, saying he wasn't aware anything had actually been taken.
The fire went to a fourth alarm, indicating it was a serious incident, a fire service spokeswoman said. No one was injured.
Earlier in the night, firefighters were called to a house fire in Tawa, just north of Wellington city.
Six appliances attended. There were no reports of injuries.
Around 3am, three Wellington fire appliances and a rural unit were called to a cow shed that had caught fire with young calves inside.
No calves were injured, a fire service spokeswoman said.