Hundreds of people have shared the news and memories on a post on the Kiwis in London Facebook page.
Kiwis in London said the Redback was "probably the original Kiwi bar here well known for cheap as drinks, hooking up, so much mayhem".
"Reckon hundreds of couples have this bar to blame for getting together!
"Kiwis used to travel for hours from all over London to drink at the Reddie until it closed in Feb 2012 ending an era of snakebite induced hangovers!"
Acton police station, next door to the pub in High St, was evacuated as a precautionary measure.
Musician Matt Blair said he was rescued from the roof of the building after his performance with double-act Rayguns Look Real Enough.
Last night the old Redback Tavern (now known as The Aeronaut)
burned down... heaps of Kiwis from decades ago remember...
Posted by Kiwis in London on Sunday, 1 January 2017
He tweeted: "We're fine but 2017 has already got off to a bad start. One of our favourite venues that we just played burned down. Everyone is fine.
"Manic. We were upstairs, had to get on a roof area and luckily found a ladder we could climb down. Scary stuff.
"It was pretty scary and shocking. Physically we're all fine. Just a bit shocked and sad."
Blair said he had heard "some idiot" used a candle to light a branch on a Christmas tree but he was unable to verify this.
The pub was hosting a "circus spectacular" with 10 separate acts to see in 2017.
The London Fire Brigade (LFB) said it rescued six people from the first floor flat while 72 firefighters worked to dampen the flames, which were brought under control by 5am.
A spokesman said: "The first and second floors, including the roof of the three-storey building, have been gutted by the fire."
Scotland Yard said three officers from the station were treated for smoke inhalation after they rushed to help.
The Metropolitan Police said the cause was not believed to be suspicious while the LFB said it was unknown.
The London Ambulance Service said it took five patients to hospital suffering from "minor injuries".
- Telegraph, staff reporter