Crews from three stations — Dunedin City, Willowbank and Roslyn —were called to the corner of lower Stuart St and Moray Pl from 8.20am, a Fire Service spokesman said.
The "full evacuation" had several streets cordoned off to protect the public, he said.
Senior station officer Rob Torrance, of Dunedin City, said people working on an upper level of a building in Stuart St smelt gas and called for firefighters.
The firefighters discovered the smell was coming from Cafe Rue in Moray Pl.
A staff member from the restaurant had left a gas hob on in the restaurant in Moray Pl overnight.
"It was human failure. I’ve talked to the owner and they are quite robust about how they check things and I guess it was just missed."
The gas detected in the restaurant was at the lowest end of "explosive range", he said.
Six firefighters wearing breathing apparatus installed two fans in the restaurant to clear the gas.
"It took about 30 or 40 minutes."
Cafe Rue owner Gary Lee said procedures were in place for staff to shut equipment down at the end of the night, including the gas hobs.
During the check on Wednesday night, the knob operating the hob appeared to be turned off but it was not.
The failure was "human" rather than mechanical because the knob had not been checked correctly, he said.
The staff member who made the "mistake" had worked for him for more than 10 years, he said.
"We’ve been here 14 years and never had anything like that happen before."
