"It's a tapu place, why would someone do this to it."
Bodies lying inside the building were fine and had been transported to Aroha Funeral Homes' other sites in Meadowbank and Takanini, Mikkelsen said.
Police are treating the circumstances surrounding the fire as suspicious.
"They took off our security system, alarms and switched off the main power," Mikkelsen said.
A message on the Facebook page for Aroha Funerals said: "We are devastated to tell you someone broke into our cottage last night and set two large fires.
"We will not be working out of there for a while, so will be doing office work from Meadowbank and have viewings at Takanini."
Staff said they were "very sad" but would keep going in a bid to provide services to the community.