Controversial pest control poison 1080 was stored in a Levin depot that was burgled and torched last week.
The fire tore through the Horizons Regional Council depot on Thursday night, destroying equipment and melting vehicles.
Fire services put out the blaze, but police said there were also signs of a burglary.
Horizons Regional Council chief executive Michael McCartney said the council was alerted to the fire early on Friday morning.
Manawatu Detective Sergeant Philip Skoglund said someone had cut through the yard's wire fence the same night, and the building had been forced open.
"There looks like there may have been some items removed but we are not sure what they are," he said.
Skoglund said the burglar may have forced access to a commercial fridge but that was yet to be determined.
"At this point it looks to be suspicious," he said.
A neighbour said he believed that the refrigerator contained 1080 but, after questions from the Horowhenua Chronicle, McCartney confirmed on Tuesday 1080 was held in a secure pesticide store in the building, though not in the fridge.
He said Horizons was still taking an inventory of items after the fire, but "all run-off associated with putting out the fire was contained onsite and there are no environmental or health concerns".
Skoglund said police were talking to the Horizons fire security manager and the last worker on the premises before the burglary.
"There was a fire around the area of a single quad bike inside the building," he said.
"We are yet to link the burglary with the quad bike that was set on fire."
McCartney said it was not yet clear how much the arson would cost ratepayers but said the council was insured."There has been considerable damage. Fortunately, our boat that undertakes monitoring on Lake Horowhenua was not onsite and will be able to continue business as usual," he said.
Three pest control staff work out of the Levin depot. The depot is also used for storage.