A Fire and Emergency New Zealand spokesperson said crews were alerted to the "well-involved" structure fire at 11.47pm on Sunday.
The Caen St blaze lasted nearly four hours as crews from the Oamaru, Weston and Glenavy fire brigades put the fire out. They had left by 3.45am.
Fire and Emergency New Zealand Waitaki group manager Mike Harrison said the building sustained "significant damage".
"It was a very intense fire and the crews worked really well together to bring it under control.
"We weren’t sure if there were people in there or not, so we treated it like there was but through the police we found out the building was unoccupied."
Fire investigators were on site earlier this week to determine the cause of the fire.
Mr Harrison encouraged anyone with any information to contact police.
