Emergency services were called to a property at Windellama, south of the NSW town of Goulburn, about 11.20am on Sunday.
Local officers were told a boy had been injured after a firearm unintentionally went off.
Paramedics treated the nine-year-old for serious neck injuries but he died at the scene.
Goulburn MP Wendy Tuckerman said the region was hurting after the youngster's death.
"This is an unspeakable tragedy and my heart goes out to the family and loved ones affected - the entire Goulburn community will be feeling this loss," she said.
"As this matter is now the subject of an active police investigation, it would be inappropriate to comment further, but I want to thank our emergency services for their swift and professional response."
Police set up a strike force to investigate the incident and later arrested a 14-year-old boy and a 33-year-old man.
The teen was charged with possessing an unauthorised firearm, investigators said on Monday.
He was granted conditional bail to appear at a children's court on May 16.
The man was charged with allowing an unauthorised person to possess a firearm and failing to keep a firearm safely.
He is due to appear before Goulburn Local Court on May 14.
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