The Masterton man and a friend were at Richmond drinking about midnight on April 8 when they entered the public toilet to escape the rain and get warm.
The summary states the pair removed the toilet roll holder and set it alight in an attempt to keep themselves warm before leaving without dousing the flames that caused substantial damage to the building.
Lawyer Ian Hard said while it was an "extremely stupid thing to do", his client did check there was no one else in the building at the time.
"The risk to life was probably to their own life," he said.
Judge Barbara Morris said there was always a danger to life with fire.
"There is always a danger with arson; the fire could spread and [a danger to] fire officers who arrive to put the fire out," she said.
Along with the six months' supervision, Duggan was ordered to pay $800 reparation to the Tasman District Council for the damage caused to the toilet block.