Aging equipment was likely to blame for the mini-blackout that left more than 200,000 Torontonians without power in the sweltering heat on Monday, says Toronto Hydro.

The outage, which encompassed large swaths of the city, started around 4:45 p.m. ET as the city was under an extreme heat alert. Traffic was plunged into chaos as lights went out and hundreds of people spilled into the streets at the height of the city's afternoon rush hour.

The blackout was caused by a fire at the Manby transformer station on Kipling Avenue in the Etobicoke section of Toronto. Hydro One, which supplies power to distributor Toronto Hydro, is investigating the cause of the fire.

As the temperature climbed into the mid-30s on Monday, so did the use of air conditioners, leading to questions about whether the increased demand on the power grid was to blame in part for the fire.

"We don't think it was related to demand. It was likely due to aging equipment or some type of failure," Toronto Hydro spokeswoman Tanya Bruckmueller told CBC Radio's Metro Morning on Tuesday.

She said Toronto Hydro is investing almost $300 million to upgrade the system over the next few years. Parts of the system were first built in the early 1950s.

Ontario Energy and Infrastructure Minister Brad Duguid said that when the Liberals came to power in 2003, they inherited a power system that was "in absolute chaos and we've made continual investments. Frankly we've been criticized for making those investments by opposition who said we shouldn't have been doing that."

Rishi Ghuldu, a pedestrian who helped direct traffic during the power outage in Toronto on Monday, speaks to a police officer. (Peter Jones/Reuters)

He dismissed notions that the government wasn't moving fast enough on upgrading the system.

"We're investing in the energy system more than anybody has in generations," he said. "And there's a cost to doing that. Our hydro bills are starting to go up now as a result of these investments."

Be ready for small outages

The demand on the power grid is likely to stay high in the coming weeks: meteorologists have predicted a hot summer for Ontario. An extreme heat alert was declared in Toronto on Monday and remained in effect Tuesday.

Torontonians should be prepared for small, localized outages as temperatures stay high, "but hopefully nothing like the one we saw yesterday," said Bruckmueller.

"Realistically, heat waves are tough on any grid," she said.

Early Tuesday, residents in north Toronto had already experienced two small outages.

About 1,100 Toronto Hydro customers in North York were affected.

One area was bounded by Steeles Avenue West to the north, Dufferin Street to the east, Finch Avenue West to the south and Jane Street to the west.

The other affected area is defined by Steeles Avenue West in the north, Rowntree Road to the south, Islington Avenue to the east and Emily Avenue to the west.

As of 7:30 a.m., power had been restored to those areas, said Bruckmueller.

The latest outage was not believed to be related to Monday's mini-blackout.