Residents in the area surrounding the bus depot, which is fenced off at night, were concerned about the incident and were interested in finding out what had happened, she said.
"There is a close proximity between the residential addresses and the buses, and it could have had a catastrophic outcome."
NZ Bus chief operating officer Rachel Drew said at least four buses had been written off.
"Three of them were brand new, only three to six months old. The other ones are only a couple of years old."
A fifth bus, which was parked behind the bus police believe was set on fire, could probably be salvaged, Ms Drew said.
While the fire and resulting damage had been hugely disappointing and frustrating, Ms Drew said she was relieved surrounding houses had not been damaged.
There had been no previous problems with security at the Darwin St site, she said.