Husbands's lawyers are arguing that extending his parole ineligibility to beyond 25 years would be cruel and unusual punishment.
This is the only the third time in Canada — and the first time in Ontario — that the new law is being considered.
Christopher Husbands was convicted in December of two counts of second-degree murder in the June 2, 2012, shooting that killed Ahmed Hassan and Nixon Nirmalendran.
Husbands fired 14 shots in the food court of the busy mall, sending hundreds fleeing.
Husbands, 25, was also found guilty of five counts of aggravated assault, one count of criminal negligence causing bodily harm and one count of reckless discharge of a firearm.
The judge has the discretion to set his parole eligibility somewhere between 10 and 25 years.
With cases involving multiple murders, sentences are usually served simultaneously.
But in this case the Crown wants the sentences served consecutively, which would leave Husbands eligible for parole only after 40 years in prison.
His lawyer Dirk Derstine is arguing that this would be unconstitutional.
"The fact that you don't have the opportunity to go back and say 'Listen, I am no longer a danger, I am a good person.' It's not the question of will they be released, the question is: Do they have the hope that they will receive some sort of due process for their request?"
Under Canadian law, Husbands can appeal his sentence if he believes it's too harsh.
When the sentencing hearing resumes today, a man who is out on parole and a forensic psychiatrist are expected to testify for the defence.