Police received the results this week from forensic testing at the Perth St address, a family home that was destroyed in a fire last month.
These tests had indicated that there was "no accelerant apparent", Detective Sergeant Paul Strange, of Oamaru, said.
Early theories indicated that an accelerant had been used to start the fire, but the forensic testing along with information from the public had led him to believe the fire was not arson, he said.
"At this stage it’s determined there was no criminal activity," he said.
He encouraged anyone who may not have come forward with any relevant information to do so.
He would await the final fire investigator’s report, which was due early next year, before making any final determination, Det Sgt Strange said.