Black smoke billowed from first-floor windows as nine fire crews battled this afternoon's blaze in the Molesworth St pub, opposite Parliament.
The fire started about 3.15pm and firefighters took about 15 minutes to bring it under control.
It was the second recent blaze the political landmark has seen, and came as workmen repaired damage from a major fire in June.
Senior station officer for Wellington Central Fire Station Brett Lockyer said a firefighter had received minor injuries when he fell through the floorboards as he entered the building.
Today's blaze was thought to have been started by sparks from cutting equipment, as two contractors worked to remove brewing tanks from the basement.
"At the early stages we believe it was started from welding, through reconstruction work that was going on from the earlier fire at the Backbencher," Mr Lockyer said.
"The fire was started in that first floor area ... [and] seems to have spread down to the basement area.
"The fire has been located and detained in the first floor and ground floor area.
He said all people had been accounted for.
It was estimated the damage to the building covered an area of 10 square metres.
Mr Lockyer said they were monitoring the building after the fire to ensure sparks had not further ignited under floorboards.
Pub owner Alistair Boyce said he was determined to reopen the building early next year.
He said a big space had been cleared so the contractors could extract the 1200-litre steel tanks but it was a difficult working environment.
"They were welding, so there's sparks ... We cut a big hole so they could get these things out, but obviously the sparks caught somewhere ... and it just got out of control."
Contractor, Kerry McDonald, said they were cutting up some distilleries when some of the sparks started the fire.
"It was such a confined space, and the vision was quite limited. We just saw some gradual smoke and then out of nowhere, just 'boom!'. We just came straight outside."
Mr McDonald was uninjured but his face and arms were blackened by smoke.
The June fire gutted the kitchen and damaged the bar and restaurant of the pub, which was famed for a collection of Spitting Image-style political puppets.
A puppet of Peter Dunne was destroyed, but puppets of Prime Minister John Key and his deputy Bill English survived.
It was believed at the time the pub would be closed for a month, but repairs have taken longer.
Mr Boyce said the damage from today's fire was not as bad as from the first.
"We just don't want it to hold us back. We want to open in February."
