Brok Hughes, 26, had been due to stand trial in October charged with murdering Zane Bradbury but pleaded guilty in the Supreme Court on Wednesday to the lesser charge of manslaughter.

After initially claiming the boy was jumping on a bed and hit his head at the couple's Glenroy home, Hughes confessed to throwing the boy.

He told police he hit the toddler and threw him onto a bed by the back of his neck to punish him for pulling down a curtain. The boy hit his head on the bed frame and the wall.

Hughes pleaded guilty to one count of intentionally causing injury to Zane between April 29 and May 9, 2014; one count of intentionally causing injury to Zane's younger brother, Chase, on May 9, 2014; and one count of killing Zane on May 9.

Justice Lex Lasry remanded Hughes for a pre-sentence hearing on April 24 and ordered the three-week trial set down for October 26 be vacated.

Police claimed Hughes waited 40 minutes before calling an ambulance to help the unconscious, seriously injured little boy.

Zane was rushed to the Royal Children's Hospital but later died from his injuries.