Firefighters ''attacked'' the blaze with two hoses before entering the building, wearing breathing apparatus, to extinguish the fire.
The fire was under control in 20 minutes, he said.
The fire had ''completely gutted'' the first floor office and the floor below was water damaged, including the kitchen and bar.
During the fire, 18 firefighters attended in five trucks, he said.
East Otago Southland fire risk management officer Cathy Trevathan said the cause had not been determined and the possibility the fire was suspicious had not been discounted.
Fish Hook owner Mike Trewern said he saw 6m flames licking out of three windows in London St and ''it was quite spectacular''.
He believed the office was above the bar's kitchen.
''They [the firefighters] got there in the nick of time before it got a hold,'' Mr Trewern said.