"It's our livelihood. We'll take it step by step: get assessors in here, get the fire department to determine exactly what the cause was."
The fire appeared to have started in the front kitchen but it was not yet known how, Mr Upton said.
The kitchen was considerably damaged and the dining room had smoke damage.
"We will do everything in our power to get the restaurant up and running again as soon as possible," Mr Upton said.
"We regret any inconvenience to our customers and we will be in touch with people who have made bookings during the weeks we are carrying out renovations."
O'Connell St Bistro, on the corner of Shortland and O'Connell Streets, has won a host of industry awards and garnered rave reviews from international publications including The New York Times.
The intimate European-inspired eatery is known for its refined menu and private dining room, where prime ministers, royalty and celebrities have enjoyed meals away from the public eye.
Fire safety officers are investigating the cause of the blaze.
