The City of Saint John confirmed the explosion in a Facebook post, noting that emergency responders were on the scene.
"Several contractors are being treated for non-life-threatening injuries in relation to this incident".
CBC News reports people nearby reported a "bed shaking explosion" and police are urging people to avoid the area. AT, and said flames and smoke could be seen from a great distance from the refinery, on the city's east side.
"There was quite a shockwave when the blast happened", he said as he left with co-workers hours later.
Tom Kloza, global head of energy analysis at Oil Price Information Service told CNBC 'We probably import more gasoline from that single refinery than any other single refinery around the world, ' he said.
The thick, black plume of smoke earlier in the day had been cause for concern for Gordon Dalzell, who lives in the nearby subdivision of Chaplain Heights - but the bigger issue, he said, is the other pollutants in the air that can be neither seen nor smelled.
No flames were visible by 2:30 p.m. Ambulances could be seen coming and going, but without their emergency signals.
There were as many as 3,000 workers on the refinery Monday - but most of the facility was shut down for major maintenance, and almost all of the workers were contractors working on the turn-around.
The refinery is near several residential neighbourhoods about five kilometres from the city core, known as Uptown. "Praying for the safety of all involved".
Irving said Monday morning that a " major incident" had occurred at its Saint John refinery.
Irving Oil is a major supplier of oil to the northeastern part of the United States.
"Our refinery opened in 1960, on a 780-acre site and has been upgraded throughout the years".
The company shut most of the plant for planned maintenance in late September, but the smaller, 25,000 bpd gasoline-making units and the 125,000 bpd crude unit were still running, a source familiar with the plant's operations said.
"There was thick, pitch black smoke mixed with white smoke", said Guimond.
Mark Melanson and Jocelyn Legassie were sitting at the Tim Hortons on Hickey Road, about three kilometres away from the refinery, when they heard a popping sound.
He made a decision to drive by the refinery and said he was nearby when he felt the vibrations of what he assumes must have been a second explosion.
The blast was believed to be the result of a malfunction in the diesel refining section of the Irving Oil refinery in St John's, New Brunswick, said company executive Kevin Scott.
They looked outside the window and saw black smoke and flames exceeding the height of the stacks at the refinery.
He says he saw hundreds of residents watching the situation unfold from a distance.