Toronto Fire say the blaze at 650 Parliament St. this week happened because items left on a stove in one of the units caught fire when the power turned back on.
The highrise has been evacuated since August because of a massive electrical fire that pushed 1,500 residents out of their homes.
Deputy Fire Chief Jim Jessop has described the damage as "catrastrophic," with crews from multiple agencies working around the clock to repair.
"Given what happened and the damage that the fire caused to the actual system, I'm not surprised that it has taken this long where they've just starting now to be able to start re-energizing the system," he said.
The work includes getting into walls, removing all electrical components and rewiring them, with Jessop adding the fire marshall has still not been able to pinpoint the exact cause.
"The crux of the work and the complexity of the work is the fact that in reality, they're re-installing and rebuilding an entire electrical system for a highrise apartment buildling," he said, on top of smoke damage.
As for reentry, a spokesperson for the building's property management company Wellesley-Parliament Square, Danny Roth, said the date is now past the summer.
"It is looking like later this fall, later in the year," he said.
Roth said the challenge is a class action lawsuit filed in February by some of the tenants over contents in units.
The company sought permission to remove the contents and place them in a secure location, but the lawsuit claims no permission was granted to box them in the first place.
"We still don't have a decision," Roth said, adding according to the company's construction team, if they aren't moved, that will add months to re-entry.