Emergency services arrived at the property at 12.50pm yesterday to find the top storey well alight.
They also discovered two people with critical and serious injuries.
A police spokesman said a man aged in his 30s was taken into custody without incident and received treatment at hospital for an injury to his head.
He had since been charged with wounding with intent to cause grievous bodily harm, and wilfully setting fire to property knowing danger to life was likely to ensue.
He was due to appear in the Timaru District Court today.
Fire and Emergency NZ returned to the scene with three fire appliances today after smoke started billowing out of the collapsed roof.
All firefighters were made to sign in with police, as they were now entering a crime scene.
Police were inspecting the site with a drone.
A fire investigator said he could not comment on the cause of the fire as it was a crime scene.
A Fenz spokesman said four tankers and four trucks attended the blaze yesterday.
