On Saturday, police were called to a building at 120 Dallimore Circle, near Lawrence Avenue East and the Don Valley Parkway.
"They received a radio call for a child that was under the water in the pool," Det. Darren Berezowski told CBC News on Sunday.
Berezowski said that "eight or nine adults," were having a barbecue at the building, along with a similar number of children who were on the patio and in the pool areas.
Seven children were in the pool at the time of the incident. No adults were present when the three-year-old fell into the water, though Berezowski said adults had been in and out of the area during the party.
"The child slipped under the water and he was underwater for approximately three minutes before an adult came into the pool, saw that he was in the water, jumped in and pulled him out," Berezowski said.
A police officer who arrived on scene started CPR on the child, a process that continued when paramedics and firefighters arrived shortly thereafter.
The child was rushed to Toronto East General Hospital, but could not be saved.
Berezowski said that the child’s mother was on the patio — just a short distance from the pool — at the time of the incident.
The detective said that he would not be laying charges against the deceased child’s parents.
At least 86 people have drowned in Ontario this year, said Barbara Byers of the Lifesaving Society.
Byers told CBC News children are often attracted to water, despite the dangers.
"For parents with young children, toddlers, you need to keep them within your eyesight, you need to be within arm's reach and your eyes on them at all times," she said.
Berezowski said that "it only takes a second for tragedy to strike and that's what this is — a tragedy."