Detectives remained at the Lalor Park house on Monday morning. Credit:Nick Moir

The girl was inside the Danny Road house with her mother and three young brothers at the time. Police said Mr Moussa was nearby.

Mr Moussa, from Arncliffe, was charged on Monday with possessing an unregistered and prohibited weapon and failing to take all reasonable precautions to ensure it was safely kept.

He was also charged with contravening an AVO that banned him from entering any premises where the protected person lives, works or attends for the purposes of childcare.

Quakers Hill Local Area Commander Superintendent Paul Carrett said the girl's parents and siblings were assisting police.

Forensic police examined the house on Danny Road in Lalor Park after a girl was shot dead. Credit:Nick Moir

"The death of a three-year-old child in any circumstances is tragic," he said. "Tragic for the family, for police and other emergency services that attend these scenes."

Superintendent Carrett highlighted the shooting as a timely reminder for people to keep firearms stored safely.

Two men sit in the gutter after a girl was shot dead in Lalor Park. Credit:TNV

Mr Moussa appeared before Blacktown Local Court on Monday. He did not apply for bail and it was formally refused.

He is scheduled to appear before the same court again on Friday.

Neighbours described the traumatic scenes that unfolded as horrified relatives emerged from the house screaming and crying on Sunday night.

"I just heard the gun go off and I came outside to see what was happening," neighbour Mark Tupa said on Monday.

"The father came out and was screaming and crying. I saw another guy leave the house with blood all over his hands."

Mr Tupa said the girl's mother came out of the house shortly after, screaming at one of the men.

"She was just saying it was his fault and he shouldn't have had a gun in the house," he said.

"I seen these three little boys run out ... they were just crying and screaming for help," Mr Tupa said.

Footage taken shortly after showed a man sitting in the gutter rocking back and forth and wailing. He was later filmed being led to the back of a police van.

The girl's mother, who was screaming and crying, was led into an ambulance.

Once inside the ambulance, she shouted: "I hate you, f--- you, I hate you!"

She could also be heard screaming: "Not my baby girl, not my baby girl!" and "Rot in hell."

A man at the scene was heard shouting: "My niece just died."

Emergency services were responding to a report of what the caller said was "a domestic".

They had been called to the address in September last year, responding to a report of multiple gunshot sounds and yelling.

Forensic teams spent the night in the house and were working at the scene early on Monday morning.

Another neighbour said the scene was chaotic with the little girl's mother and father distraught.

With AAP