Officers attended an address in Spring Street, Concord, at 12.30am on Saturday following reports of a stabbing.
They arrived to find a man with multiple stab wounds to his chest lying in the driveway.
The 15-year-old boy was being questioned by investigators over the stabbing. (Bianca De Marchi/AAP PHOTOS)
The man, who is yet to be formally identified, was treated by paramedics and received CPR but died at the scene.
Police confirmed they had seized a weapon they allege was used to kill the man, saying it was "more than a kitchen knife".
"Today, one life was lost and one life will be ruined," Superintendent Christine McDonald told reporters.
"It is why we have laws against young people carrying knives on our streets.
"One day, in the spur of the moment, you may use that knife; you may kill someone with that knife.
"And if that day comes, you will likely be facing a murder charge and jail time."
The 15-year-old boy was being questioned by investigators at Burwood Police Station on Saturday but was yet to be charged.
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