Police were called to a unit in Sydney's Hills District about 5pm on Monday, September 21, following reports of a shooting.
The 12-year-old boy, one of 10 people at the Norwest unit at the time, was found with gunshot wounds to his face and neck.
He was treated at the scene before being rushed to hospital in a critical condition.
He died in hospital three days later.
Three 18-year-old men were arrested following the incident, including two who were released without charge pending further inquiries.
The third was charged with concealing a serious indictable offence and remained in custody.
That same man was arrested at a correctional facility in Silverwater, in Sydney's west, on Tuesday and charged with murder.
Homicide squad detectives laid the fresh charge after taking carriage of the investigation following the boy's death.
The accused, who is now 19, was remanded in custody and is expected to face court on Wednesday.
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