Early on November 14, a house bus parked in Maitland St, Dunedin, burst into flames.
Fire and Emergency New Zealand investigator Matt Jones, along with the police criminal investigation branch and a police photographer, investigated due to the "suspicious nature" of the fire.
He ultimately determined it was deliberately lit.
Mr Jones said the fire was caused "by persons unknown and the ignition source was either taken away or consumed in the fire".
"Fire patterns within the wheel arch space area also supported a hypothesis that a fire was started with an ignition source that could not be identified."
The inside of the bus had almost no fire damage apart from a storage area below seats on the passenger side, Mr Jones said.
The bus, a 1989 Mitsubishi Rosa camper, had not been used for some time.
Electrical, gas, wiring and mechanical problems were quickly ruled out.
"Three hypotheses were developed from the information and evidence collected during the scene examination.
"The separate hypotheses were researched and tested against the fire patterns and physical evidence found at the scene.
"The results from the testing eliminated two hypothesis as causing the fire."
