On Monday morning Lees, who taught at the Anglican Church Grammar School - where Brad was a preschooler - drove from his home in the southeastern suburb of Seven Hills to Kangaroo Pt, where the 74m-high Story Bridge crosses the Brisbane River to Fortitude Valley.
He left his car, strapped his son into the baby seat on his bicycle and rode onto the bridge before throwing the boy into the river and leaping after him.
Witness Craig Cooper told Fairfax media he had watched in horror from his nearby balcony.
"I just heard this scream. It was blood-curdling. I don't think I could describe it, it was unbelievable.
"I caught a glimpse of a falling thing, it was tiny. A few seconds later he [Lees] jumped. It was unmistakable this time that it was a body."
Fairfax reported that Lees was believed to have recently separated from his psychologist wife, Danielle.
Neighbours told News Ltd that the family had appeared to be happy.
"He was a lovely boy," one said of the dead toddler. "He would wave to you if you saw them out the front."
Another said they had been a lovely family: "I guess [Danielle Lees] will just have to try to put her life back together. It's just her now."
Anglican Grammar headmaster Jonathan Hensman told the Courier Mail that Lees had been an outstanding and popular teacher.
"Jason was a highly regarded and much-loved teacher ... [Brad] was a lovely boy and that just adds to the tragedy."